ADA4940-1ACPZ-R7 datasheet(27/30 Pages) AD

27 / 30 page

Data Sheet

ADA4940-1/ADA4940-2

Rev. D | Page 27 of 30

DRIVING A HIGH PRECISION ADC

The ADA4940-1/ADA4940-2 are ideally suited for broadband

dc-coupled applications. The circuit in Figure 73 shows a front-

end connection for an ADA4940-1 driving an AD7982, which is

an 18-bit, 1 MSPS successive approximation, analog-to-digital

converter (ADC) that operates from a single power supply, 3 V

to 5 V. It contains a low power, high speed, 18-bit sampling

ADC and a versatile serial interface port. The reference voltage,

REF, is applied externally and can be set independent of the

supply voltage. As shown in Figure 73, the ADA4940-1 is dc-

coupled on the input and the output, which eliminates the need

for a transformer to drive the ADC. The amplifier performs a

single-ended-to-differential conversion if needed and level

shifts the input signal to match the input common mode of the

ADC. The ADA4940-1 is configured with a dual 7 V supply

(+6 V and −1 V) and a gain that is set by the ratio of the

feedback resistor to the gain resistor. In addition, the circuit

can be used in a single-ended-input-to-differential output or

differential-input-to-differential output configuration. If needed,

a termination resistor in parallel with the source input can be

used. Whether the input is a single-ended input or differential,

the input impedance of the amplifier can be calculated as shown in

the Terminating a Single-Ended Input section. If R1 = R2 = R3 =

R4 = 1 kΩ, the single-ended input impedance is approximately

1.33 kΩ, which, in parallel with a 52.3 Ω termination resistor,

provides a 50 Ω termination for the source. An additional 25.5 Ω

(1025.5 Ω total) at the inverting input balances the parallel

impedance of the 50 Ω source and the termination resistor driving

the noninverting input. However, if a differential source input is

used, the differential input impedance is 2 kΩ. In this case, two

52.3 Ω termination resistors are used to terminate the inputs.

In this example, the signal generator has a 10 V p-p symmetric,

ground-referenced bipolar output. The VOCM input is bypassed for

noise reduction and set externally with 1% resistors to 2.5 V to

maximize the output dynamic range. With an output common-

mode voltage of 2.5 V, each ADA4940-1 output swings between

0 V and 5 V, opposite in phase, providing a gain of 1 and a

10 V p-p differential signal to the ADC input. The differential RC

section between the ADA4940-1 output and the ADC provides

single-pole, low-pass filtering with a corner frequency of 1.79 MHz

and extra buffering for the current spikes that are output from the

ADC input when its sample-and-hold (SHA) capacitors are

discharged.

The total system power in Figure 73 is under 35 mW. A large

portion of that power is the current coming from supplies to the

output, which is set at 2.5 V, going back to the input through the

feedback and gain resistors. To reduce that power to 25 mW,

increase the value of the feedback and gain resistor from 1 kΩ

to 2 kΩ and set the value of the resistors R5 and R6 to 3 kΩ. The

ADR435 is used to regulate the +6 V supply to +5 V, which ends

up powering the ADC and setting the reference voltage for the

VOCM pin.

Figure 72 shows the fft of a 20 kHz differential input tone

sampled at 1 MSPS. The second and third harmonics are down

at −118 dBc and −122 dBc.

–160

–140

–120

–100

–80

–60

–40

–20

20k

40k

60k

80k

100k

FREQUENCY (Hz)

Figure 72. Distortion Measurement of a 20 kHz Input Tone (See CN-0237)

33Ω

10µF

–DIN

+2.5V

+5V

+6V

–1V

+6V

REF

VDD

GND

IN+

IN–

AD7982

2.7nF

–IN

+OUT

–OUT

+IN

+DIN

ADR435

0.1µF

SERIAL

INTERFACE

–FB

+FB

ADA4940-1

VOCM

Figure 73. ADA4940-1 (LFCSP) Driving the AD7982 ADC

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