DC1287A - LINEAR TECHNOLOGY

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1287A

LTC6416 2 GHZ LOW NOISE DIFFERENTIAL 16-BIT ADC BUFFER

DESCRIPTION

Demonstration circuit 1287 is a 2 GHz Low Noise Differ-

ential 16-Bit ADC Buffer featuring the LTC6416.

It incor-

porates a variety of passive components to support

configurations for varied applications. Both single-

ended and differential configurations are supported

at the inputs and at the outputs.

The LTC6416 is a high-speed differential buffer am-

plifier with superior distortion and noise perform-

ance. It is intended for driving high speed 14 and 16

bit ADCs. It is perfect in applications such as de-

manding communications transceivers, cellular base

stations, and other high-speed signal chain applica-

tions.

Notable features of the LTC6416 include high input

impedance, adjustable output voltage limiting and

simple output common-mode adjustment. The

LTC6416’s high input impedance allows it to be used

50Ω systems with

1:4 and 1:8 input transformers

for additional system gain. The VCLLO and VCLHI

pins set lower and upper limits on the output voltages

seen at each differential output pin. This feature helps

protect the ADC inputs from undervoltage and over-

voltage waveforms. The output common-mode volt-

age of the LTC6416 is determined by the voltage ap-

plied to the VCM pin. It is internally buffered then

drives the LTC6416’s differential inputs through 2kΩ

series resistors. For AC-Coupled operation, no exter-

nal bias network is needed. DC-coupled inputs applied

to the LTC6416 can easily override the 2kΩ resistance

of the internal bias network. Design files for this cir-

cuit board are available. Call the LTC factory.

, LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered

trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy

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UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names

may be trademarks of the companies that manufacture the products.

QUICK START PROCEDURE

Table 1 shows the function of each SMA connector on

the board.

Table 1: DC1287 SMA Connector Descriptions

J1 (IN+)

Single-Ended/Differential Input.

By default, this is

configured as a single-ended input.

Use this con-

nector to supply an input to the DC1287. Drive

from a 50ohm signal source, no external termina-

tion necessary.

J2 (IN-)

Differential Input. Not connected by default. Ca-

pacitor C10 can be installed and C12 removed to

drive the DC1287 differentially.

J3 (OUT+)

Single-Ended/Differential Output.

By default, this is

configured as a single-ended output.

This output is

designed to drive 50 ohms, such as a network or

spectrum analyzer input.

J4 (OUT-)

Differential Output. Not connected by default. Ca-

pacitor C9 can be installed and C11 removed for

differential output drive.

Figure 1 shows the Top Silkscreen for the DC1287

Demo Board.

Figure 1: DC1287 Top Silkscreen

DEMO

CIRCUIT

1287

QUICK START GUIDE

LTC6416

2 GHz Low Noise Differential 16-

Bit ADC Buffer

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1287A

LTC6416 2 GHZ LOW NOISE DIFFERENTIAL 16-BIT ADC BUFFER

Refer to Table 1 and Figure 1, and follow the proce-

dure below:

Connect ground to both GND turrets and a 3.6V

power supply to the V+ turret. VCM, CLLO and

CLHI turrets may be left floating.

Apply an input signal to J1. It should be >10MHz

because of the lower frequency limit of the 1:1 in-

put and output transformers. A good starting

point for amplitude is about 0dBm (0.63V

) at the

DC1287A’s input. A low-distortion, low noise sig-

nal source with an external high-order low-pass or

band-pass filter will yield the best performance.

The DC1287’s input is impedance-matched to

50Ω; no external termination is necessary.

Observe the output via J3. Although the LTC6416

is intended to drive the high-impedance inputs of

an ADC, the DC1287A’s output has been designed

to drive 50Ω, suitable for the input of a network or

spectrum analyzer.

A resistive network comprised

of R1, R3, 18

Ω of internal resistance and the 50Ω

load terminates the LTC6416 in a 400Ω differen-

tial load. T2, a 1:1 balun, transforms the output

signal for single-ended observation. Due to volt-

age division within the output network, an at-

tenuation of approximately -19dB is observed at

the output.

ADDITIONAL INFORMATION

Although the DC1287A is ready to use out of the box,

it has features that you can access by adding, remov-

ing or changing components on the board. These are

described below.

DIFFERENTIAL INPUTS

To drive the DC1287A differentially, remove capacitor

C12 and install capacitor C10. J1 and J2 can now be

driven differentially from a 50Ω signal source. The

input to the DC1287A will still be matched to 50Ω

(differential) when driven in this manner.

DIFFERENTIAL OUTPUTS

To use the both outputs of the DC1287A, remove

capacitor C11 and install capacitor C9. The output

now appears differentially across J3 and J4. The

output of the DC1287A will still be matched to 50Ω

(differentially) when used in this manner.

CHANGING THE OUTPUT COMMON-MODE

VOLTAGE

Turret E1 (labeled VCM) controls the output com-

mon-mode voltage of the DC1287A. This function

can be used to level-shift the DC output voltage for

optimum system performance. Normally, capacitors

C8 and C9 prevent the output common mode voltage

from reaching the output. However, if the common

mode voltage is desired, C8 and C9 can be shorted

or replaced by 0Ω resistors to allow the DC bias to

reach the output. Transformer T2 is a 1:1 transmis-

sion line type, which means DC voltages will couple

from input to output of the transformer. By default,

the LTC6416 will self-bias the VCM pin to 1.36V

(with V+=3.6V). For a different common-mode volt-

age, simply apply a voltage source to Turret E1.

ADJUSTING THE CLLO AND CLHI VOLTAGES

The LTC6416 features high speed voltage limiting at each

of the output pins. This feature is important to help pro-

tect the inputs of the ADC. The output voltage at either of

the differential outputs will not exceed the voltage set at

the LTC6416’s CLHI pin. Similarly, it will not go below

the voltage set at the LTC6416’s CLLO pin. See the

LTC6416 datasheet for more information.

Turret E4 (labeled CLHI) drives the CLHI pin of the

LTC6416 directly. If left floating, the CLHI pin will self-

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1287A

LTC6416 2 GHZ LOW NOISE DIFFERENTIAL 16-BIT ADC BUFFER

bias to 2.45V (with V+=3.6V). For a different CLHI volt-

age, simply apply a voltage source to Turret E4.

Turret E5 (labeled CLLO) drives the CLLO pin of the

LTC6416 directly. If left floating, the CLLO pin will self-

bias to 0.265V (with V+=3.6V). For a different CLLO volt-

age, simply apply a voltage source to Turret E5.

OUTPUT DRIVE

Because the output of the LTC6416 is a low-

impedance voltage output, performance will degrade

when the output is directly driving low impedances

such as 50Ω. See the LTC6416 datasheet for more

information.

GROUND AND V+ CONNECTIONS

DC1287A’s GND connector is not only connected to

GND of the LTC6416, it also serves as the board

ground. For best performance, a low-impedance re-

turn path to the power supply from GND is crucial.

Short, low impedance wires to the VCC and GND

connectors of DC1287A will yield the best perform-

ance from the LTC6416.

CABLES AND CONNECTORS

Long BNC or SMA cables are transmission lines, and

for best circuit performance should be kept as short

as possible. BNC or SMA cables or barrels should be

characterized (or nulled out through calibration) for

good high-frequency performance. Connectors also

fall under the category of circuit elements, and must

be of good quality and well characterized to ensure

predictable results.

SIGNAL LEVELS

If we examine the schematic included with this Quick

Start Guide, we can determine approximate signal

levels seen at various points along DC1287A’s signal

chain. These signal levels assume that we start with

0dB at J1, and that there is a 50Ω load at the out-

put(s). If the output load is a high-impedance load,

the signal levels will vary from the schematic.

With 0dB at J1, 1:1 transformer T1 performs a sin-

gle-ended to differential conversion. There is about

0.5dB of attenuation in the transformer, so the differ-

ential signal level at the inputs of the LTC6416 is

about -0.5dB. The differential signal level at the out-

puts of the LTC6416 is about -1dB. The output resis-

tor network results in an additional 17.5dB of at-

tenuation at the input of 1:1 transformer T2 (-18.5dB

total). Transformer T2 performs a differential to sin-

gle-ended conversion and adds a final 0.5dB of at-

tenuation bringing the total attenuation to -19dB.

The external series resistors at the LTC6416’s output

have been included so that the output of the

DC1287A can plug directly into a 50Ω analyzer input,

but the LTC6416 sees a more benign 400Ω load re-

sistance.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1287A

LTC6416 2 GHZ LOW NOISE DIFFERENTIAL 16-BIT ADC BUFFER