THAT2181BL08-U - THAT CORPORATION

88Output

77V+

66Gnd

55V-

44Sym

33Ec-

22Ec+

11Input

SO PinSIP PinPin Name

Table 1. Pin Assignments

Figure 1. 2181 Series Equivalent Circuit Diagram

FEATURES

• Wide Dynamic Range: >120 dB

• Wide Gain Range: >130 dB

• Exponential (dB) Gain Control

• Low Distortion:

~ 0.0025 % (typical 2181A)

~ 0.005 % (typical 2181C)

• Wide Gain-Bandwidth: 20 MHz

• Dual Gain-Control Ports (pos/neg)

• Pin-Compatible with 2150-Series

APPLICATIONS

• Faders

• Panners

• Compressors

• Expanders

• Equalizers

• Filters

• Oscillators

• Automation Systems

THAT 2181A, 2181B, 2181C

THAT 2181 Series integrated-circuit voltage

controlled amplifiers (VCAs) are very high-

performance current-in/current-out devices with

two opposing-polarity, voltage-sensitive control

ports. They offer wide-range exponential control

of gain and attenuation with low signal distortion.

The parts are selected after packaging based

primarily on after-trim THD and control-voltage

feedthrough performance.

The VCA design takes advantage of a fully

complementary dielectric isolation process which

offers closely matched NPN/PNP pairs. This deliv-

ers performance unobtainable through any

conventional process, integrated or discrete. The

parts are available in three grades, allowing the

user to optimize cost vs. performance. Both 8-pin

single-in-line (SIP) and surface mount (SO)

packages are available.

Description

Blackmer®

rimmable

oltage Controlled Amplifiers

THAT Corporation; 45 Sumner

Street; Milford,

assachusetts

01757-1656; US

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

2181CS08-U2181CL08-U0.05% 2181BS08-U2181BL08-U0.02% 2181AS08-U2181AL08-U0.01%

Plastic

SIP

Max Trimmed

THD

@1V,1kHz,0dB

Table 2. Ordering information

BIAS CURRENT

COMPENSATION

Vbe

MULTI-

PLIER Output

Sym

Iset

V-

Vcc

Ec+

Ec-

Iadj

Input

Gnd

Document 600030 Rev 02 Page 2

of 12 THAT 2181

Series

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

Positive Supply Voltage (VCC)+20 V

Negative Supply Voltage (VEE) -20 V

Supply Current (ICC) 10 mA

Maximum ∆EC EC+ - (EC-) ± 1 V

Power Dissipation (PD) (TA = 75 ºC) 330 mW

Operating Temperature Range (TOP) 0 to +70 ºC

Storage Temperature Range (TST) -40 to +125 ºC

Absolute Maximum Ratings2,3

SPECIFICATIONS1

2181A 2181B 2181C

Parameter Symbol Conditions Min Typ Max Min Typ Max Min Typ Max Units

Supply Current ICC No signal — 2.4 4 — 2.4 4 — 2.4 4 mA

Equiv. Input Bias Current IB No Signal — 2 10 — 2 12 — 2 15 nA

Input Offset Voltage VOFF(IN) No Signal — ±5 — — ±5 — — ±5 — mV

Output Offset Voltage VOFF(OUT) Rout = 20 kΩ

0 dB gain — 0.5 1 — 1 2 — 1.5 3 mV

+15 dB gain — 1 3 — 1.5 4 — 3 10 mV

+30 dB gain — 3 12 — 5 15 — 9 30 mV

Gain Cell Idling Current IIDLE —20 — —20 — —20 — µA

Gain-Control Constant TA =25°C (TCHIP≅35°C)

-60 dB < gain < +40 dB

EC+ /Gain (dB) Pin 2 (Fig. 15) 6.0 6.1 6.2 6.0 6.1 6.2 6.0 6.1 6.2 mV/dB

EC- /Gain (dB) Pin 3 -6.2 -6.1 -6.0 -6.2 -6.1 -6.0 -6.2 -6.1 -6.0 mV/dB

Gain-Control TempCo ∆EC /∆TCHIP Ref TCHIP = 27°C — +0.33 — — +0.33 — — +0.33 — %/°C

Gain-Control Linearity -60 to +40 dB gain — 0.5 2 — 0.5 2 — 0.5 2 %

1 kHz Off Isolation EC+= -360mV,EC-=+360mV 110 115 — 110 115 — 110 115 — dB

Output Noise en(OUT) 20 Hz ~ 20 kHz

Rout = 20kΩ

0 dB gain — -98 -97 — -98 -96 — -98 -95 dBV

+15 dB gain — -88 -86 — -88 -85 — -88 -84 dBV

Voltage at V- VV- No Signal -3.1 -2.85 -2.6 -3.1 -2.85 -2.6 -3.2 -2.85 -2.6 V

Electrical Characteristics2

1. All specifications are subject to change without notice.

2. Unless otherwise noted, TA=25ºC, VCC=+15V, VEE= -15V. Test circuit as shown in Figure 2. SYM ADJ is adjusted for minimum THD at 1 V, 1 kHz, Ec- = -Ec+ = 0 V.

3. Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only; the functional operation of

the device at these or any other conditions above those indicated in the operational sections of this specification is not impli ed. Exposure to absolute maximum rating condi-

tions for extended periods may affect device reliability.

2181A 2181B 2181C

Parameter Symbol Conditions Min Typ Max Min Typ Max Min Typ Max Units

Positive Supply Voltage VCC +4 +15 +18 +4 +15 +18 +4 +15 +18 V

Negative Supply Voltage VEE -4 -15 -18 -4 -15 -18 -4 -15 -18 V

Bias Current ISET VCC - VEE = 30 V 1 2.4 3.5 1 2.4 3.5 1 2.4 3.5 mA

Signal Current IIN + IOUT ISET = 2.4 mA — 0.35 2.5 — 0.35 2.5 — 0.35 2.5 mA

Recommended Operating Conditions

THAT 2181 Series Page 3

of 12 Documen

600030

Rev 02

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

2181A 2181B 2181C

Parameter Symbol Conditions Min Typ Max Min Typ Max Min Typ Max Units

Total Harmonic Distortion THD 1 kHz

VIN = 0 dBV, 0 dB gain — 0.0025 0.005 — 0.004 0.008 — 0.005 0.02 %

VIN = +10 dBV, -15 dB gain — 0.018 0.025 — 0.025 0.035 — 0.035 0.07 %

VIN = -5 dBV, +15 dB gain — 0.018 0.025 — 0.025 0.035 — 0.035 0.07 %

VIN = +10 dBV, 0 dB gain — 0.004 0.008 — 0.006 0.010 — 0.015 — %

Slew Rate RIN = ROUT = 20 kΩ—12 — —12 — —12 —V/µs

Symmetry Control Voltage VSYM AV = 0dB, Minimum THD -0.5 — +0.5 -1.5 — +1.5 -2.5 — +2.5 mV

Gain at 0 V Control Voltage EC- = 0 mV -0.1 0.0 +0.1 -0.15 0.0 +0.15 -0.2 0.0 +0.2 dB

Electrical Characteristics (con’t)2

Vcc Ec-

10u 20k

5.1k

Vee

OUT

22p

20k

OUT

Vcc

Vee

50k SYM

ADJ

Rsym

OP275

V+

-IN Ec-

Ec+

SYM

GND

V-

2181

Series

VCA

Power Supplies

Vcc = +15 V

Vee = -15 V

680k (2181A)

220k (2181B)

130k (2181C)

Figure 2. Typical Application Circuit

Figure 3. 2181 Series Frequency Response vs. Gain Figure 4. 2181 Series Noise (20kHz NBW) vs. Gain

The THAT 2181 Series VCAs are designed for

high performance in audio-frequency applications

requiring exponential gain control, low distortion,

wide dynamic range and low control-voltage

feedthrough. These parts control gain by converting

an input current signal to a bipolar logged voltage,

adding a dc control voltage, and re-converting the

summed voltage back to a current through a bipolar

antilog circuit.

Figure 5 presents a considerably simplified inter-

nal circuit diagram of the IC. The ac input signal

current flows in pin 1, the input pin. An internal

operational transconductance amplifier (OTA) works

to maintain pin 1 at a virtual ground potential by

driving the emitters of Q1 and (through the Voltage

Bias Generator) Q3. Q3/D3 and Q1/D1 act to log the

input current, producing a voltage, V3, which repre-

sents the bipolar logarithm of the input current. (The

voltage at the junction of D1 and D2 is the same as

V3, but shifted by four forward Vbe drops.)

Gain Control

Since pin 8, the output, is usually connected to a

virtual ground, Q2/D2 and Q4/D4 take the bipolar

antilog of V3, creating an output current which is a

precise replica of the input current. If pin 2 (EC+) and

pin 3 (EC-) are held at ground (with pin 4 - SYM -

connected to a high impedance current source), the

output current will equal the input current. For pin 2

positive or pin 3 negative, the output current will be

scaled larger than the input current. For pin 2

negative or pin 3 positive, the output current is

scaled smaller than the input.

In normal operation, the output current is

converted to a voltage via an opamp-based I-V

converter, as shown in Figure 2, where the conver-

sion ratio is determined by the feedback resistor

connected between the output and inverting input.

The signal path through the VCA and the output

opamp is non-inverting.

The scale factor between the output and input

currents is the gain of the VCA. Either pin 2 (Ec+) or

pin 3 (Ec-), or both, may be used to control gain.

Gain is exponentially proportional to the voltage at

pin 2, and exponentially proportional to the negative

of the voltage at pin 3. Therefore, pin 2 (Ec+) is the

positive control port, while pin 3 (Ec-) is the

Document 600030 Rev 02 Page 4

of 12 THAT 2181

Series

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

Figure 5. Simplified Internal Circuit Diagram

Figure 6. Gain vs. Control Voltage (EC+, Pin 2) at 25°C

Theory of Operation4

Figure 7. Gain vs. Control Voltage (EC-, Pin 3) at 25°C

Figure 8. Gain vs. Control Voltage (EC-) with Temp (°C)

4. For more details about the internal workings of the 2181 Series of VCAs, see An Improved Monolithic Voltage-Controlled

mplifier, by Gary K. Hebert (Chief Technology Officer, for THAT Corporation), presented at the 99th convention of the

udio Engineering Society, New York, Preprint number 4055

negative control port. Because of the exponential

characteristic, the control voltage sets gain linearly

in decibels. Figure 6 shows the decibel current gain

of a 2181 versus the voltage at Ec+, while Figure 7

shows gain versus the Ec-.

Temperature Effects

The logging and antilogging in the VCA depends

on the logarithmic relationship between voltage and

current in a semiconductor junction (in particular,

between a transistor's Vbe and IC). As is well known,

this relationship is temperature dependent. There-

fore, the gain of any log-antilog VCA depends on its

temperature.

Figure 8 shows the effect of temperature on the

negative control port. (The positive control port

behaves in the same manner.) Note that the gain at

Ec = 0 V is 0 dB, regardless of temperature. Chang-

ing temperature changes the scale factor of the gain

by 0.33%/°C, which pivots the curve about the 0 dB

point.

Mathematically, the 2181's gain characteristic is

, Eq. 1

Gain =EC+−EC−

(0.0061)(1+0.0033T)

where ∆T is the difference between room

temperature (25°C) and the actual temperature, and

Gain is the gain in decibels. At room temperature,

this reduces to

, Eq. 2

Gain =EC+−EC−

0.0061

If only the positive control port is used, this

becomes

, Eq. 3

Gain =EC+

0.0061

If only the negative control port is used, this

becomes

, Eq. 4

Gain =EC−

0.0061

DC Bias Currents

The 2181 current consumption is determined by

the resistor between pin 5 (V-) and the negative

supply voltage (VEE). Typically, with 15V supplies, the

resistor is 5.1 kΩ, which provides approximately

2.4 mA. This current is split into two paths: 570 μA

is used for biasing the IC, and the remainder

becomes ICELL as shown in Figure 5. ICELL is furth

er split in two parts: about 20 μA biases the core

transistors (Q1 through Q4), the rest is available for

input and output signal current.

Trimming

The 2181-Series VCAs are intended to be

adjusted for minimum distortion by applying a small

variable offset voltage to pin 4, the SYM pin. Note

that there is a 25 Ω resistor internal to the 2181

between pin 4 and pin 2. As shown in Figure 2,

Page 3, the usual method of applying this offset is to

use the internal 25 Ω resistor along with a larger

value resistor to form a voltage divider connected to

the wiper of a trim pot across the supply rails.

This trim should be adjusted for minimum

harmonic distortion. This is usually done by applying

a middle-level, middle-frequency signal (e.g. 1 kHz at

1 V) to the audio input, setting the VCA to 0 dB gain,

and adjusting the SYM trim while observing THD at

the output. In the 2181, this adjustment coincides

closely with the setting which produces minimum

control-voltage feedthrough, though the two settings

are not always identical.

DC Feedthrough

Normally, a small dc error term flows in pin 8

(the output). When the gain is changed, the dc term

changes. This control-voltage feedthrough is more

pronounced with gain; the –A version of the part

produces the least feedthrough, the –C version the

most. See Figure 9 for typical curves for dc offset vs.

gain

Audio Performance

The 2181-Series VCA design, fabrication and

testing ensure extremely good audio performance

when used as recommended. The 2181 maintains

low distortion over a wide range of gain, cut and

signal levels. Figures 10 through 12 show typical

distortion performance for representative samples of

each grade of the part. At or near unity gain, the

2181 behaves much like a good opamp, with low

distortion over the entire audio band. Figure 13

shows typical THD for a 2181A over frequency at 0

dB gain, with a 1 V input signal, while Figure 14

details the harmonic content of the distortion in a

typical A–grade part.

THAT 2181 Series Page 5

of 12 Documen

600030

Rev 02

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

Figure 9. Representative DC Offset vs. Gain

Document 600030 Rev 02 Page 6

of 12 THAT 2181

Series

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

Figure 13. 2181A THD+N vs Frequency, 0dB gain 1kHz 1V

Figure 14. FFT of THD, Typical 2181A,

0dB Gain, 1V, 1kHz

Figure 10. 1 kHz THD+Noise vs. Input Level, 0dB Gain

Figure 11. 1 kHz THD+Noise vs. Input Level, +15dB Gain

Figure 12. 1 kHz THD+Noise vs. Input Level, -15dB Gain

Input

As mentioned above, input and output signals are

currents, not voltages. While this often causes some

conceptual difficulty for designers first exposed to

this convention, the current input/output mode

provides great flexibility in application.

The Input pin (pin 1) is a virtual ground with

negative feedback provided internally (see Figure 5,

Page 4). The input resistor (shown as 20 kΩ in

Figure 2, Page 3) should be scaled to convert the

available ac input voltage to a current within the

linear range of the device. Generally, peak input

currents should be kept under 1 mA for best distor-

tion performance.

Refer to Figures 10 through 12 to see how distor-

tion varies with signal level for the three parts in the

2181 Series for 0 dB, +15 dB and -15 dB gain. The

circuit of Figure 2, Page 3 was used to generate these

curves.

For a specific application, the acceptable distor-

tion will usually determine the maximum signal

current level which may be used. Note that, with

20 kΩ current-to-voltage converting resistors, distor-

tion remains low even at 10 V rms input at 0 dB or

-15 dB gain, and at 1.7 V rms input at +15 dB gain

(~10 V rms output). This is especially true in the –A

and –B grades of the part.

Distortion vs. Noise

A designer may trade off noise for distortion by

decreasing the 20 kΩ current-to-voltage converting

resistors used at the input and output in Figure 2,

Page 3. For every dB these resistor values are

decreased, the voltage noise at the output of the

OP275 is reduced by one dB. For example, with

10 kΩ resistors, the output noise floor drops to

–104 dBV (typical) at 0 dB gain — a 6 dB reduction

in noise because 10 kΩ is 1/2 of (6 dB lower than)

20 kΩ.

Conversely, if THD is more important than noise

performance, increasing these resistors to 40 kΩ will

increase the noise level by 6 dB, while reducing

distortion at maximum voltage levels. Furthermore, if

maximum signal levels are higher (or lower) than the

traditional 10 V rms, these resistors should be

scaled to accommodate the actual voltages prevalent

in the circuit. Since the 2181 handles signals as

currents, these ICs can even operate with signal

levels far exceeding the 2181's supply rails, provided

appropriately large resistors are used.

High-Frequency Distortion

The choice of input resistor has an additional,

subtle effect on distortion. Since the feedback imped-

ances around the internal opamp (essentially Q1/D1

and Q3/D3) are fixed, low values for the input resis-

tor will require more closed-loop gain from the

opamp. Since the open-loop gain naturally falls off at

high frequencies, asking for too much gain will lead

to increased high-frequency distortion. For best

results, this resistor should be kept to 10 kΩ or

above.

Stability

An additional consideration is stability: the inter-

nal op amp is intended for operation with source

impedances of less than 60 kΩ at high frequencies.

For most audio applications, this will present no

problem.

DC Coupling

The quiescent dc voltage level at the input (the

input offset voltage) is approximately 0 V, but, as in

many general-purpose opamps, this is not well

controlled. Any dc input currents will cause dc in the

output which will be modulated by gain; this may

cause audible thumps. If the input is dc coupled, dc

input currents may be generated due to the input

offset voltage of the 2181 itself, or due to offsets in

stages preceeding the 2181. Therefore, capacitive

coupling is almost mandatory for quality audio appli-

cations. Choose a capacitor which will give accept-

able low frequency performance for the application.

Summing Multiple Input Signals

Multiple signals may be summed via multiple

resistors, just as with an inverting opamp configura-

tion. In such a case, a single coupling capacitor may

be located next to pin 1 rather than multiple capaci-

tors at the driven ends of the summing resistors.

However, take care that the capacitor does not pick

up stray signals.

Output

The Output pin (pin 8) is intended to be

connected to a virtual ground node, so that current

flowing in it may be converted to a voltage (see

Figures 2 & 15). Choose the external opamp for good

audio performance. The feedback resistor should be

chosen based on the desired current-to-voltage

conversion constant. Since the input resistor deter-

mines the voltage-to-current conversion at the input,

THAT 2181 Series Page 7

of 12 Documen

600030

Rev 02

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

Applications

the familiar ratio of Rf /Ri for an invertin

opamp will

determine the overall voltage gain when the 2181 is

set for 0 dB current gain. Since the VCA performs

best at settings near unity gain, use the input and

feedback resistors to provide design-center gain or

loss, if necessary.

A small feedback capacitor around the output

opamp is needed to cancel the output capacitance of

the VCA. Without it, this capacitance will destabilize

most opamps. The capacitance at pin 8 is typically

15 pF.

Power Supplies

Positive

The positive supply is connected directly to V+

(pin 7). No special bypassing is necessary, but it is

good practice to include a small (~1 μf) electrolytic

or (~0.1 μf) ceramic capacitor close to the VCA IC on

the PCB. Performance is not particularly dependent

on supply voltage. The lowest permissible supply

voltage is determined by the sum of the input and

output currents plus ISET , which must be supplied

through the output of the internal transconductance

amplifier and down through the core and voltage bias

generator. Reducing signal currents may help accom-

modate low supply voltages. THAT Corporation

intends to publish an application note covering

operation on low supply voltages. Please inquire for

its availability.

The highest permissible supply voltage is fixed by

the process characteristics and internal power

consumption. +18 V is the nominal limit.

Negative

The negative supply terminal is V- (pin 5). Unlike

normal negative supply pins, this point is intended to

be connected to a current source ISET (usually simply

a resistor to VEE), which determines the current avail-

able for the device. As mentioned before, this source

must supply the sum of the input and output signal

currents, plus the bias to run the rest) of the IC. The

minimum value for this current is 570 μA over the

sum of the required signal currents. Usually, ISET

should equal 2.4 mA for most pro audio applications

with ±15 V supplies. Higher bias levels are of limited

value, largely because the core transistors become

ineffective at logging and antilogging at currents over

1 mA.

Mathematically, this can be expressed as

ICELL ≥ Peak (IIN) + Peak (IOUT) + 220 μA; and

ICELL = ISET - 350 μA. Therefore,

ISET ≥ Peak (IIN) + Peak (IOUT) + 570 μA.

The voltage at V- (pin 5) is four diode drops

below ground, which, for the 2181, is approximately

-2.85 V. Since this pin connects to a (high

impedance) current supply, not a voltage supply,

bypassing at pin 5 is not normally necessary.

Ground

The GND pin (pin 6) is used as a ground refer-

ence for the VCA. The non-inverting input of the

internal opamp is connected here, as are various

portions of the internal bias network. It may not be

used as an additional input pin.

Voltage Control

Negative Sense

EC- (pin 3) is the negative voltage control port.

This point controls gain inversely with applied

voltage: positive voltage causes loss, negative voltage

causes gain. As described on Page 5, the current gain

of the VCA is unity when pin 3 is at 0 V with respect

to pin 2, and varies with voltage at approximately

-6.1 mV/dB, at room temperature.

Positive Sense

As mentioned earlier, EC+ (pin 2) is the positive-

sense voltage control port. A typical circuit using this

approach is shown in Figure 15. EC- (Pin 3) should be

grounded, and EC+ (pin 2) driven from a

low-impedance voltage source. Using the opposite

sense of control can sometimes save an inverter in

the control path.

Positive and Negative

It is also possible (and sometimes advantageous)

to drive both control ports, either with differential

drive (in which case, the control sensitivities of each

port are summed), or through two different control

signals. There is no reason why both control ports

cannot be used simultaneously.

Document 600030 Rev 02 Page 8

of 12 THAT 2181

Series

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

Symmetry

The SYM pin (pin 4) is actually a sort of

additional positive-sense control port. It is provided

to allow Vbe mismatches in the core transistors to be

adjusted after packaging and installation in the

circuit board. It should only be used for this

purpose. Connect pin 4 only to a high-impedance

source as shown in Figures 2 and 15.

Control Port Drive Impedance

The control ports (pins 2 through 4) are

connected directly to the bases of the logging and/or

antilogging transistors. The accuracy of the logging

and antilogging is dependent on the EC+ and EC-

voltages being exactly as desired to control gain. The

base current in the core transistors will follow the

collector currents, of course. Since the collector

currents are signal-related, the base currents are

therefore also signal-related. Should the source

impedance of the control voltage(s) be large, the

signal-related base currents will cause signal-related

voltages to appear at the control ports, which will

interfere with precise logging and antilogging, in turn

causing distortion.

The 2181 Series VCAs are designed to be

operated with zero source impedance at pins 2 and

3, and a high (≥50 kΩ) source impedance at pin 4.

To realize all the performance designed into a 2181,

keep the source impedance of the control voltage

driver well under 50 Ω.

This often suggests driving the control port

directly with an opamp. However, the closed-loop

output impedance of an opamp typically rises at hi

frequencies because open loop gain falls off as

frequency increases. A typical opamp's output imped-

ance is therefore inductive at high frequencies.

Excessive inductance in the control port source

impedance can cause the VCA to oscillate internally.

In such cases, a 100 Ω resistor in series with a

1.5 nF capacitor from the control port to ground will

usually suffice to prevent the instability.

Noise Considerations

It is second nature among good audio designers

to consider the effects of noisy devices on the signal

path. As is well known, this includes not only active

devices such as opamps and transistors, but extends

to the choice of impedance levels as well. High value

resistors have higher inherent thermal noise, and the

noise performance of an otherwise quiet circuit can

be easily spoiled by the wrong choice of impedance

levels.

Less well known, however, is the effect of noisy

circuitry and high impedance levels in the control

path of voltage-control circuitry. The 2181 Series

VCAs act like multipliers: when no signal is present

at the signal input, noise at the control input is

rejected. So, when measuring noise (in the absence

of signal – as most everyone does), even very noisy

control circuitry often goes unnoticed. However,

noise at the control port of these parts will cause

noise modulation of the signal. This can become

significant if care is not taken to drive the control

ports with quiet signals.

THAT 2181 Series Page 9

of 12 Documen

600030

Rev 02

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

Vcc Ec-

10u 20k

5.1k

Vee

OUT

22p

20k

OUT

Vcc

Vee

50k SYM

ADJ

Rsym

OP275

V+

-IN Ec-

Ec+

SYM

GND

V-

2181

Series

VCA

Power Supplies

Vcc = +15 V

Vee = -15 V

680k (2181A)

220k (2181B)

130k (2181C)

Figure 15. Positive Control Port using Pin 4

The 2181 Series VCAs have a small amount o

inherent noise modulation because of its class AB

biasing scheme, where the shot noise in the core

transistors reaches a minimum with no signal, and

increases with the square root of the instantaneous

signal current. However, in an optimum circuit, the

noise floor rises only to -94 dBV with a 50 μA rms

signal at unity gain — 4 dB of noise modulation. By

contrast, if a unity-gain connected, non-inverting

5534 opamp is used to directly drive the control

port, the noise floor will rise to 92 dBV — 6 dB of

noise modulation.

To avoid excessive noise, one must take care to

use quiet electronics throughout the control-voltage

circuitry. One useful technique is to process control

voltages at a multiple of the eventual control constant

(e.g., 61 mV/dB — ten times higher than the VCA

requires), and then attenuate the control signal just

before the final drive amplifier. With careful attention

to impedance levels, relatively noisy opamps may be

used for all but the final stage.

Stray Signal Pickup

It is also common practice among audio design-

ers to design circuit boards to minimize the pickup

of stray signals within the signal path. As with noise

in the control path, signal pickup in the control path

can adversely effect the performance of an otherwise

good VCA. Because it is a multiplier, the 2181

produces second harmonic distortion if the audio

signal itself is present at the control port. Only a

small voltage at the control port is required: as little

as 10 μV of signal can increase distortion to over

0.01%. This can frequently be seen at high frequen-

cies, where capacitive coupling between the signal

and control paths can cause stray signal pickup.

Because the signal levels involved are very small,

this problem can be difficult to diagnose. One clue to

the presence of this problem is that the symmetry

null for minimum THD varies with frequency. It is

often possible to counteract a small amount of pure

fundamental picked up in the control path by

"misadjusting" the symmetry setting. Since the

amount of pickup usually varies with frequency, the

optimum trim setting will vary with frequency and

level. A useful technique to confirm this problem is

to temporarily bypass the control port to ground via

a modest-sized capacitor (e.g., 10 μF). If the distor-

tion diminishes, signal pickup in the control path is

the likely cause.

Temperature Sensitivity

As shown by Equation 1 (Page 5), the gain of a

2181 VCA is sensitive to temperature in proportion

to the amount of gain or loss commanded. The

constant of proportionality is 0.33% of the decibel

gain commanded, per degree Celsius, referenced to

27°>C (300°K). This means that at 0 dB gain, there

is no change in gain with temperature. However, at

-122 mV, the gain will be +20 dB at room tempera-

ture, but will be 20.66 dB at a temperature 10°C

lower.

For most audio applications, this change with

temperature is of little consequence. However, if

necessary, it may be compensated by a resistor

embedded in the control voltage path whose value

varies with temperature at the same rate of 0.33%/°C.

Such parts are available from RCD Components, Inc,

www.rcd-comp.com, and KOA/Speer Electronics,

www.koaspeer.com.

Closing Thoughts

THAT Corporation welcomes comments,

questions and suggestions regarding these devices,

their design and application. Our engineering staff

includes designers who have decades of experience in

applying our parts. Please feel free to contact us to

discuss your applications in detail.

Document 600030 Rev 02 Page 10

of 12 THAT 2181

Series

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

The THAT 2181-series is available in 8-pin SO and

8-pin SIP packages. Package dimensions are shown

in Figure 16 and 17 below; Pinouts are given in

Table 1 on page 1. Ordering information is provided

in Table 2 also on page 1.

The 2181-series packages are entirely lead-free.

The lead-frames are copper, plated with successive

layers of nickel, palladium, and gold. This approach

makes it possible to solder these devices using lead-

free and lead-bearing solders.

Neither the lead-frame nor the plastic mold

compound used in the 2181-series contains any

hazardous substances as specified in the European

Union's Directive on the Restriction of the Use of

Certain Hazardous Substances in Electrical and

Electronic Equipment 2002/95/EG of January 27,

2003

THAT 2181 Series Page 11

of 12 Documen

600030

Rev 02

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

Parameter Symbol Conditions Min Typ Max Units

Through-hole package See Figure 16 for dimensions 8 Pin SIP

Thermal Resistance θJA SIP package soldered to board 100 ºC/W

Environmental Regulation Compliance Complies with January 27, 2003 RoHS requirements

Surface-mount package See Figure 17 for dimensions 8 Pin SO

Thermal Resistance θJA SO package soldered to board 150 ºC/W

Soldering Reflow Profile JEDEC JESD22-A113-D (250ºC)

Moisture Sensitivity Level MSL Above-referenced JEDEC soldering profile 1

Environmental Regulation Compliance Complies with January 27, 2003 RoHS requirements

Package Characteristics

E TYP.

N 17.78 ±0.3 0.700 ±0.012

MILLIMETERS

19.5 +0.2/-0

1.25

0.65

0.85

2.54 ±0.2

0.9

1.2

5.8 +0.2/-0

2.8 +0.1/-0

10.5 ±0.5

1.3

0.3

3.5 ±0.5

INCHES

0.77 +0.008/-0

0.049

0.026

0.033

0.100 ±0.008

0.04

0.05

0.23 +0.008/-0

0.11 +0.004/-0

0.413 ±0.02

0.05

0.012

0.14 ±0.02

ITEM

Package Information

Figure 16. -L (SIP) Version Package Outline Drawing Figure 17. -S (SO) Version Package Outline Drawing

Document 600030 Rev 02 Page 12

of 12 THAT 2181

Series

Blackmer® Trimmable IC VCAs

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

Notes