DC1250A-AA - LINEAR TECHNOLOGY

16-BIT HIGH PERFORMANCE ADC DRIVERS

DESCRIPTION

Demonstration circuit 1250 is an evaluation board featur-

ing Linear Technology Corporation’s LTM9001 16-bit

Receiver Subsystem. DC1250 demonstrates good circuit

layout techniques and recommended external circuitry

for optimal system performance.

DC1250 comes installed with one of Linear Technology’s

16-bit LTM9001 amplifier/ADC subsystems. The board

includes a wideband input transformer (for evaluation

with a single-ended RF signal generator) and output

CMOS buffers.

The LTM9001 is also capable of gene-

rating LVDS outputs – for evaluation of the part with

LVDS outputs, obtain DC1241.

DC1250 plugs into the

DC890 Data Acquisition demo board and the output can

be easily analyzed with Linear Technology’s PScope data

processing software, which is available for no charge on

our website at http://www.linear.com.

Design files for this circuit board are available. Call

the LTC factory.

, LTC and LT are registered trademarks of Linear Technology Corporation.

QUICK START PROCEDURE

Validating the performance of the LTM9001 is simple

with DC1250, and requires only an input source, a clock

source, a computer, and a lab power supply. Refer to

Figure 1 for proper board evaluation equipment setup

and follow the procedure below:

Connect the power supply as shown in Figure 1. There

are on-board low-noise voltage regulators that provide

the three supply voltages for the LTM9001. The entire

board and all components share a common ground.

The power supply should still be a low-noise lab pow-

er supply capable of supplying at least 1 Amp.

Provide an encode clock to the ADC via SMA connec-

tor J3. Use a low-phase-noise clock source such as a

filtered RF signal generator or a high-quality clock os-

cillator.

Obtain DC1216 for a low-phase-noise ADC

clock source that can plug directly into DC1250.

NOTE.

Similar to having a noisy input, a high-jitter (phase noise) encode

clock will degrade the signal-to-noise ratio (SNR) of the system.

Table 1: DC1250 Connectors and Jumpers

REFERENCE FUNCTION

J1 (AIN+) Board Signal Input. Impedance-matched to

for use with lab signal generators.

J2 (AIN-) Differential Board Input.

Normally not con-

nected.

See text for differential-input evalua-

tion methods.

J3 (ENC) Board Clock Input. Impedance-matched to

Ω.

Drive with a low-phase-noise clock oscil-

lator or filtered sine wave signal source.

E1 (EXT REF) Reference input to adjust the full-scale range of

the LTM9001. Connects to the SENSE pin; by

default, tied to VDD for internal reference.

JP5 (AMP_EN) Enables the LTM9001 amplifier. Default is EN.

JP1 (PGA_GAIN) Selects the input range of LTM9001. Default is

LOW (low PGA gain, larger input range)

JP2 (RAND) Output Randomizer. NORM is default.

JP3 (ADC_SHDN) Enables the LTM9001 ADC. Default is NORM.

JP4 (DITH) ADC Internal Dither. Default is OFF.

Apply an input signal to the board. DC1250 allows

great flexibility in applying input signals (see the sec-

tion on Applying Input Signals). For best results, use a

low distortion, low noise signal generator with suffi-

cient filtering to avoid degrading the performance of

the amplifier and ADC.

Observe the ADC output with demo circuit DC890, a

USB cable, a Windows computer, and Linear Technol-

ogy’s Pscope data processing software.

DEMO CIRCUIT 1250

QUICK START GUIDE

LTM9001 16-bit Receiver

Subsystem

LTM9001 16-BIT RECEIVER SUBSYS

Figure 1.

Proper Evaluation Equipment Setup

ADDITIONAL INFORMATION

Although the DC1250 demo board is ready to use on de-

livery, it has additional flexibility built in for various types

of input networks. Below is some information about con-

figuring DC1250 to meet the specific needs of your eval-

uation.

APPLYING INPUT SIGNALS

The input network consists of various components de-

signed to allow either single-ended or differential inputs,

AC-coupled or DC-coupled. Table 2 shows some possi-

ble input configurations, and which components to in-

stall. LTM9001 is designed for excellent performance

with both single-ended and differential input drive, with

little difference in distortion performance. When using

DC-coupled inputs, the inputs to DC1250 need to be lev-

el-shifted to within the input common-mode limits in the

datasheet.

Table 2: DC1250 Input Configuration Guide

CONFIGURATION COMPONENTS NECESSARY

Single-Ended Input

AC-Coupled

(Default Setup)

No change. Transformer T1 acts as a balun for

differential drive.

Single-Ended Input

No Transformer

AC-Coupled

Remove T1, replace with 0

jumpers. May need

to install impedance-matching resistor at R4

or R1/R7.

Single-Ended Input

No Transformer

DC-Coupled

Same as above. Change C1 and C8 to 0

jum-

pers. Inputs must be within the common-

mode voltage limits of LTM9001.

Differential Inputs Remove R7 and install R5. T1 and C1/C8 can be

replaced with 0

for DC coupling.

NOTE.

When driving the ADC driver with a direct DC-coupled path, increased

input bias currents may occur due to the amplifier’s input impedance. See

the LTM9001 datasheet for more details.

Signal

Generator

HP 8644B

or equiv.

RF Signal Generator

(HP 8644B) or other

low phase noise

clock source (e.g. DC1216)

Power Supply

(3.8V-6V @ 1.0A)

To DC890

BPF

NOTE.

Even a high-quality signal

synthesizer will still have noise and

harmonics that should be attenuated

with a low-pass or band-pass filter.

For good-quality high order filters,

see TTE, Lark Engineering, or

equivalent.

BPF

LTM9001 16-BIT RECEIVER SUBSYS

OUTPUT DATA FORMAT

The default output format for DC1250 is demultiplexed

CMOS output mode, which alternates the digital outputs

coming out of the LTM9001 “A” and “B” channels. The

first output word is shifted out of the “A” channel, the

second word out of the “B” channel, and so on. Thus

each channel is updating at half the data rate.

By default,

DC890 and PSCOPE read the board in demultiplexed

CMOS mode.

Removing resistor R19 on the DC1250 would ground the

LVDS pin, and the digital outputs would all come out of

channel “A” at the full data rate. However, PSCOPE is not

currently set up to read the data this way. When using

the outputs in full-data-rate mode, you must manually

configure PSCOPE (see Figure 2) to 2-channel mode,

and ignore the “B” channel display in PSCOPE.

OTHER BOARD CIRCUITRY

Device U5 is an EEPROM that is used by the PScope

software to identify the board and apply the correct set-

tings for the data collection.

USING PSCOPE SOFTWARE

PScope, downloadable from Linear Technology’s website

http://www.linear.com/, processes data from the DC890

FastDAACS board and displays FFT and signal analysis

information on the computer screen. The on-board

EEPROM U5 should enable automatic board detection

and auto-configuration of the software, but if the user

wishes to change the settings, they can easily do so.

From the Configure menu in the toolbar, uncheck “Auto-

detect Device”. The default settings are shown in Figure

2. The LTM9001 also has an output randomizer, which

the user needs to select if it is enabled on the board (un-

der a different menu option). The software will automati-

cally un-randomize the output by performing an exclu-

sive-OR with each bit and the LSB.

Figure 2.

Entering the correct device information for your ADC. Select the correct parameters for the DC1250. Under normal conditions,

PSCOPE should automatically recognize the board and adjust the software settings accordingly.

LTM9001 16-BIT RECEIVER SUBSYS

DC1250 Schematic (Separated into 2 pages)

LTM9001 16-BIT RECEIVER SUBSYS