ADA4945-1ACPZ-R2 datasheet(42/44 Pages) AD

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ADA4945-1

Data Sheet

Rev. 0 | Page 42 of 44

DRIVING A HIGH PRECISION ADC

The ADA4945-1 is ideally suited for broadband dc-coupled

applications. The recommended list of precision converters is

shown in Table 15. The circuit in Figure 116 shows an example

of the ADA4945-1 driving a precision ADC such as the AD4003

(an 18-bit, 2 MSPS, successive approximation ADC), or the

AD7768 (a 24-bit, 256 kSPS, sigma-delta ADC). The ADA4945-1

is dc-coupled on the input and the output, which eliminates the

need for a transformer to drive the ADC. In this example, the

ADA4945-1 is applied in a differential input to differential

output configuration, with a gain of 1, and with dual supplies of

+7 V and −2 V. The output of the ADA4945-1 is level shifted to

match the input common mode of the ADC. The gain 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 configuration. If needed, a termination resistor in parallel

with the source input can be used. When a single-ended input

is used, the input impedance of the amplifier can be calculated as

shown in the Terminating a Single-Ended Input section. If the

feedback and gain resistors are all 1 kΩ, as in Figure 116, 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.

When driving the AD7768 in this example, the ADA4945-1 is

driven by a signal generator having an 8 V p-p symmetric,

bipolar output. The VOCM input of the ADA4945-1 is bypassed for

noise reduction and is driven via the common-mode source of the

AD7768 to 2.5 V. With an output common-mode voltage of 2.5 V,

each ADA4945-1 output swings between 0 V and 4 V, opposite

in phase, providing a gain of 1 and a 8 V p-p differential signal

to the ADC input. The differential RC section between the

ADA4945-1 output and the ADC input provides a single-pole,

low-pass filter to help reduce current spikes due to ADC input

switching.

Table 14 shows the SNR and total harmonic distortion (THD)

of the ADA4945-1 driving the AD7768 and AD4003 for various

input frequencies at a near full-scale signal. The RC filter values

in Figure 116 are also shown, as well as the reference voltage

(REF) level.

ADA4945-1

+7V

–2V

1kΩ

+2.5V

ADC

–IN

+IN

VREF

+VCLAMP

–VCLAMP

CDIFF

REF

+DIN

–DIN

Figure 116. ADA4945-1 Driving Precision ADC

Table 14. SNR and THD for ADA4945-1 Driving AD7768 and AD4003

ADC

Frequency (kHz)

Signal Level (V p-p)

REF (V)

R (Ω)

C (nF)

CDIFF (nF)

SNR (dB)

THD (dB)

AD7768

8.0

4.096

0.27

0.68

106.7

−115.9

8.0

4.096

0.27

0.68

106.5

−115.5

8.0

4.096

0.27

0.68

105.8

−116.9

7.98

4.096

0.27

0.68

104.7

−116.2

AD4003

9.5

5.0

200

180.0

Not applicable

98.5

−123.5

9.5

5.0

200

180.0

Not applicable

98.3

−117.0

100

9.1

5.0

200

180.0

Not applicable

96.3

−100.3

Table 15. Recommended Converters

Product

Power (mW)

Throughput (MSPS)

Resolution (Bits)

SNR (dB)

AD4001

AD4003

100

AD4005

AD4007

100

AD4011

0.5

100

AD4020

100

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