ADA4807-2ACPZ-R2 datasheet(27/33 Pages) AD

27 / 33 page

Data Sheet

ADA4807-1/ADA4807-2/ADA4807-4

Rev. B | Page 27 of 33

ADA4807-1/

ADA4807-2/

ADA4807-4

ADA4807-1/

ADA4807-2/

ADA4807-4

+5V

VOUT

27pF

2pF

20pF

VOS

TRIM

100Ω

R12

100Ω

1kΩ

5kΩ

R13

100Ω

10Ω

100Ω

5kΩ

200Ω

VOS

TRIM

V–

–5V

V+

qn0

qn1

1/2 LSK489

1/2 PMP4201

ADR510

1/2 PMP4201

1/2 LSK489

–5V

–

Figure 70. Low Noise FET Operational Amplifier Schematic

CH1 200mV

CH2 1V

M100ns

A CH1

Ω

Figure 71. Pulse Response, G = 10, 4 V p-p Output

–60

–65

–70

–75

–80

–85

–90

–95

–100

–105

–110

100

10k

20k

FREQUENCY (Hz)

Figure 72. 8 V p-p Output, THD + N for G = 10, RLOAD = 600 Ω

POWER MODE ADC DRIVER

One of the merits of a SAR ADC, such as the AD7980, is that its

power scales with the sampling rate. This power scaling makes

SAR ADCs very power efficient, especially when running at a

low sampling frequency. However, the ADC driver used with

the SAR ADC traditionally consumes constant power regardless

of the sampling frequency.

Figure 73 illustrates a method by which the quiescent power of

the ADC driver can be reduced by 95% while still maintaining

the input signal to the ADC. Both the ADA4807-1/ADA4807-2/

ADA4807-4 and the AD8603 are rail-to-rail input and output

(RRIO) amplifiers and can operate on a single 5 V analog supply.

Connecting the AD8603 in parallel with a sharing resistor allows

the ADA4807-1/ADA4807-2/ADA4807-4 to be powered down,

reducing the total supply current for the driver from 1 mA to

50 μA. The sampling frequency of the AD7980 can then be

reduced to match the power consumption of the AD8603. With

the ADA4807-1/ADA4807-2/ADA4807-4 powered on, the SNR

and THD are 84.1 dB and −100.3 dB for a 3 V p-p, 1 kHz input

and a 4.096 V reference. The SNR and THD degrade to 81.4 dB

and −77.3 dB for the same input signal in the low power mode

when only the AD8603 is on.

One issue with this method is that the reference and reference

buffer power do not scale with the ADC or the driver. This

makes this configuration most useful in multichannel systems

where the reference can be reused across many inputs.

Alternately, the reference buffer can be scaled in the same

fashion as the input driver; however, the reference itself must

remain on in any of the modes.

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