DC798B - LINEAR TECHNOLOGY - Product.Network

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DEMO MANUAL DC798B

Description

LTC2480

16-Bit Delta Sigma ADC with

Easy Drive Input Current Cancellation

Demonstration circuit 798B features the LTC2480, a 16-bit

high performance ΔΣ analog-to-digital converter (ADC).

The LTC2480 features 0.12LSB linearity, 0.16LSB full-scale

accuracy, 5μV offset, and 1.45μVRMS noise. The input is

fully differential, with input common mode rejection of

140dB. The LTC2480 is available in a 10-pin DFN package

and has an easy to use SPI interface.

DC798B is a member of Linear Technology‘s QuikEval™

family of demonstration boards. It is designed to allow easy

evaluation of the LTC2480 and may be connected directly

to the target application’s analog signals while using the

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and

QuikEval is a trademark of Linear Technology Corporation. All other trademarks are the property

of their respective owners.

BoarD photo

DC590 USB Serial Controller board and supplied software

to measure performance. The exposed ground planes allow

proper grounding to prototype circuitry. After evaluating

with Linear Technology’s software, the digital signals can

be connected to the end application’s processor/controller

for development of the serial interface.

Design files for this circuit board are available at

http://www.linear.com/demo

Figure 1. Connection Diagram

MONITOR REFERENCE OR

EXTERNAL REFERENCE SOURCE

(SEE HARDWARE SET-UP)

DIFFERENTIAL

INPUT

RIBBON CABLE

TO DC590

CONTROLLER

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DEMO MANUAL DC798B

Quick start proceDure

Figure 2. Software Screenshot

Connect DC798B to a DC590 USB Serial Controller using

the supplied 14-conductor ribbon cable. Connect DC590

to host PC with a standard USB A/B cable. Run the evalu-

ation software supplied with DC590 or downloaded from

http://www.linear.com/software. The correct program will

be loaded automatically. Click the COLLECT button to start

reading the input voltage. Details on software features are

documented in the control panel’s help menu.

Tools are available for logging data, changing reference

voltage, changing the number of points in the strip chart

and histogram, and changing the number of points aver-

aged for the DVM display.

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DEMO MANUAL DC798B

harDware setup

Connection To DC590 Serial Controller

J1 is the power and digital interface connector. Connect

to DC590 serial controller with supplied 14-conductor

ribbon cable.

Jumpers

JP1: Select the source for REF+. Tw o 5V references are

available to suit different applications—an LT1790A-5

suitable for most applications using the lower gain set-

tings and an LT1236 that allows full noise performance

at the highest gain settings. An external reference can

also be applied to the REF+ turret post if JP1 is removed

completely.

Analog Connections

Analog signal connections are made via the row of turret

posts along the edge of the board. Also, when connecting

the board to an existing circuit the exposed ground planes

along the edges of the board may be used to form a solid

connection between grounds.

GND: Three ground turrets are connected directly to the

internal ground planes.

VCC: This is the supply for the ADC. Do not draw any

power from this point.

REF+: Connected to the LTC2480 reference pin. If one of

the onboard references is being used, the reference voltage

may be monitored from this point. An external reference

may be connected to these terminals if JP1 is removed.

IN+/IN– : These are the differential inputs to the LTC2480.

f0: An external conversion clock may be applied to the f0

turret to modify the frequency rejection characteristics

or data output rate of the LTC2480. This should be a

square wave with a low level equal to ground and a high

level equal to VCC. While up to a 2MHz clock can be used,

performance may be compromised. Refer to the LTC2480

data sheet. This terminal has a 5k pull-down resistor to

keep it from floating when no signal is applied.

eXperiMents

Input Noise

One of the characteristics of the LTC2480 is that the 600nV

input noise floor is far below the quantization level of 38µV

when a 5V reference is used and the PGA is set to a gain

of 1. This means that the output will be stable if the input

noise level is significantly below 38µV. In this sense, the

LTC2480 is a true 17 effective bit part, whereas many 16-bit

SAR converters have several LSBs of noise.

Solder a short wire from the IN– turret post to the IN+

turret post. Noise should be below the quantization level

of the LTC2480. This will result in a noise reading of zero

on the control software.

Increase the gain setting to 256 (1× speed mode.) This

results in a 149nV LSB size, allowing a true measurement

of the input noise. Tw o references are included on DC798B;

an LT1790-5.0 and an LT1236-5. For applications using the

lower gain settings, the LT1790 is adequate. However, in

the highest gain settings the LT1790 can elevate the input

noise by a factor of 2 or 3. The LT1236 is significantly

quieter than the LT1790 and is a good choice for very high

resolution applications. The LT1790 is a good choice for

low power applications or where small size is required.

Common Mode Rejection

Tie the two inputs (still connected together) to ground

through a short wire and note the indicated voltage. Tie

the inputs to REF+; the difference should be less than

0.5μV due to the 140dB minimum CMRR of the LTC2480.

Bipolar Symmetry

To demonstrate the symmetry of the ADCs transfer func-

tion, connect a stable, low noise, floating voltage source

(with a voltage less than VREF /2) from IN+ to IN– and note

the indicated voltage. Reverse the polarity; the indicated

voltage will typically be within one LSB of the first reading

multiplied by –1.

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DEMO MANUAL DC798B

eXperiMents

One convenient voltage source for this experiment is a

single alkaline battery. While a battery has fairly low noise,

it is sensitive to temperature drift. It is best to use a large

(D-size) battery that is insulated from air currents. A bet-

ter source is a battery powered series reference such as

the LT1790. This part is available with output voltages of

1.25V, 2.048V, 2.5V, 3V, 3.3V, 4.096V and 5V.

Input Normal Mode Rejection

The LTC2480’s SINC4 digital filter is trimmed to reject

50Hz or 60Hz line noise when operated with the internal

conversion clock. To measure input normal mode rejec-

tion, connect IN– to a 2.5V source such as an LT1790-2.5

reference or 1k – 1k divider from the onboard 5V reference

parts List

ITEM QTY REFERENCE PART DESCRIPTION MANUFACTURER/PART NUMBER

Required Circuit Components

1 3 C1, C6, C7 CAP., X7R, 0.1µF, 16V, 20%, 0402 TDK, C1005X7R1C104M

2 2 C2, C3 CAP., X5R, 4.7µF, 6.3V, 20%, 0603 TDK, C1608X5R0J475M

3 1 C4 CAP., X5R, 1µF, 6.3V, 20%, 0402 TDK, C1005X5R0J105M

4 1 C5 CAP., X7R, 0.01µF, 25V, 10%, 0402 AVX, 04023C103KAT1A

5 8 E1 to E8 TESTPOINT, TURRET, .064" MILL-MAX, 2308-2

6 1 JP1 JMP, 3-PIN, 1 ROW, .079CC COMM-CON, 2802S-03-G1

7 1 SHUNTS FOR JP1 PIN 1 & 2 SHUNT, .079" CENTER COMM-CON CCIJ2MM-138G

8 1 J1 HEADER, 2 × 7 PIN, 0.079CC MOLEX, 87331-1420

9 4 R1, R3, R4, R5 RES., CHIP 4.99k 1/16W, 1%, 0402 AAC, CR05-4991FM

10 1 R2 RES., CHIP 100, 1/16W, 5%, 0402 AAC, CR05-101JM

11 1 U1 I.C., LTC2480CDD, DFN10DD LINEAR TECH., LTC2480CDD

12 1 U2 I.C., LT1790ACS6-5, SOT23-6 LINEAR TECH., LT1790ACS6-5

13 1 U3 I.C., 24LC025, TSSOP8 MICROCHIP, 24LC025

14 1 U4 I.C., LT1236ACS8-5, SO8 LINEAR TECH., LT1236ACS8-5

to ground. Apply a 10Hz, 2V peak-to-peak sine wave to IN+

through a 1µF capacitor. No DC bias is required because

the 2M to 3M input impedance of the LTC2480 tends to

self-bias the input to mid-reference (see data sheet Ap-

plications Information for details.)

Start taking data. The input noise will be quite large, and

the graph of output vs. time should show large variations.

Next, slowly increase the frequency to 60Hz (or 50Hz or

55Hz, depending on the selected rejection frequency.) The

noise should be almost undetectable in the graph. Note

that the indicated noise in ppm may still be above that

of the datasheet specification because the inputs are not

connected to a DC source.

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DEMO MANUAL DC798B

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

scheMatic DiagraM

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DEMO MANUAL DC798B

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com

 LINEAR TECHNOLOGY CORPORATION 2012

LT 1013 • PRINTED IN USA

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