AD9040A

REV. B

–7–

In applications using multiple AD9040As, slaving the reference

inputs to a single reference output will improve gain tracking

among the ADCs, as shown in Figure 5.

AD9040A

0.1 F

AD9040A

AD9040A

0.1 F

0.1 F

0.1 F

0.1 F

0.1 F

Figure 5. Slaving Multiple AD9040As to a Single Internal

Reference

In the specifications table, the Gain Tempco parameter under

DC ACCURACY applies to the ADC when the internal refer-

ence is being used. If an external reference is used, its tempera-

ture coefficient must be taken into account to determine overall

temperature performance.

The input range can be varied by adjusting the reference voltage

applied to the AD9040A. By decreasing the reference voltage,

the gain can be reduced approximately 10% with no degrada-

tion in performance. Increasing the reference voltage increases

the gain; but for proper operation, the reference voltage should

not exceed +2.6 V.

Time-Gain Control ADC

Ultrasound and sonar systems require an increase in gain versus

time. This allows the system to correct for attenuation of return

pulses. Figure 6 shows the AD600/AD602 amplifier and the

AD9040A ADC configured as a time-gain control analog-to-

digital converter. The control voltage ramps from –625 mV to

+625 mV, permitting 40 dB of gain-control range. The voltage

used for gain control can be either a linear ramp, or the output

of a voltage-output DAC such as the AD7242.

AD9040A

–625mV

+625mV

GAIN CONTROL

VOLTAGE

AD600/602

Figure 6.

Ultrasound/Sonar Time-Gain Control ADC

Using X-AMPs™

Transient Response

Figure 7 illustrates the method for evaluating ADC transient

performance. Two synthesizers are locked in synchronization,

but tuned to frequencies which are slightly offset from a 2-to-1

submultiple.

One synthesizer clocks a flat pulse network at a frequency of

19.9609375 MHz to provide the analog input signal; the other

synthesizer output is shaped to provide a CMOS 40 MHz sam-

pling clock. At the output of the AD9040A, output data reflects

an interleaved alias of the input pulse. The repetitive sampling

allows the measurement of ADC transient response as shown in

performance graphs elsewhere in this data sheet.

AD9040A

MARCONI 2030

SYNTHESIZER

REF

MARCONI 2030

SYNTHESIZER

REF

19.9609375 MHz

40 MHz

FLAT PULSE

NETWORK

SINE

TO

CMOS

ANALOG

IN

ENCODE

OUTPUT

Figure 7. Transient Response Test

X-AMP is a trademark of Analog Devices, Inc.