ADA4932-1/ADA4932-2

Data Sheet

Rev. E | Page 24 of 27

Another aspect of optimization is the selection of the power

supply voltages for the ADA4932-1. In the circuit, the output

common-mode voltage (VCM pin) of the AD7626 is 2.048 V

for the internal reference voltage of 4.096 V, and each input

(IN+, IN−) swings between 0 V and 4.096 V, 180° out of phase.

This provides an 8.2 V full-scale differential input to the ADC.

The ADA4932-1 output stage requires about 1.4 V headroom

with respect to each supply voltage for linear operation.

Optimum distortion performance is obtained when the supply

voltages are approximately symmetrical about the common-mode

voltage. If a negative supply of −2.5 V is chosen, then a positive

supply of at least +6.5 V is needed for symmetry about the

common-mode voltage of 2.048 V.

Experiments performed indicate that a positive supply of 7.25 V

gives the best overall distortion for a 2.4 MHz tone. Using a low

jitter clock source and a single tone −1 dBFS amplitude, 2.402 MHz

input to the AD7626 yielded the results shown in Figure 63 of

88.49 dB SNR and −86.17 dBc THD. At this input level, the ADC

limits the SFDR to 83.8 dB. As can be seen from the plot, the

harmonics of the fundamental alias back into the pass band.

For example, when sampling at 10 MSPS, the third harmonic

(7.206 MHz) is aliased into the pass band at 10.000 MHz –

7.206 MHz = 2.794 MHz.

FREQUENCY (MHz)

Figure 63. AD7626 Output, 64,000 Point, FFT Plot −1 dBFS Amplitude

2.40173 MHz Input Ton, 10.000 MSPS Sampling Rate

The nonharmonic noise admitted through the pass band of the

band-pass filter used in the circuit is replaced by the average

noise across the Nyquist bandwidth when calculating the SNR

and THD. The performance of this or any high speed circuit is

highly dependent on proper PCB layout. This includes, but is

not limited to, power supply bypassing, controlled impedance

lines (where required), component placement, signal routing,

and power and ground planes. For a more detailed analysis of

this circuit, refer to Circuit Note CN-0105.

2.4MHz

BPF

FROM

50Ω

SIGNAL

SOURCE

ADA4932-1

VCM

VDD1

VDD2

VIO

AD8031

AD7626

0.1µF

0.1µF

+5V

+5V

+2.5V

+2.5V

R3

499Ω

R5

499Ω

R2

53.6Ω

R1

53.6Ω

C1

2.2nF

R4

39Ω

0.1µF

0.1µF

0.1µF

R7

499Ω

R6

499Ω

+2.048V

1

56

7

8

–FB

2

9

+IN

3–IN

4+FB

16 15 14

13

+7.25V

–2.5V

–OUT

+OUT

PAD

R8

33Ω

R9

33Ω

11

10

C5

56pF

C6

56pF

IN–

IN+

0V TO

+4.096V

+4.096V

TO 0V

GND

0.1µF

0.1µF

0.1µF

Figure 64. ADA4932-1 Driving the AD7626 (All Connections and Decoupling Not Shown)