AD8370 datasheet(18/28 Pages) AD

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AD8370

Rev. 0 | Page 18 of 28

–100

–90

–80

–70

–60

–50

–40

326496

128

GAIN CODE

HD2

LOW GAIN MODE

HIGH GAIN MODE

HD3

Figure 48. Harmonic Distortion of the Circuit in Figure 45

DC-COUPLED OPERATION

AD8370

SERIAL CONTROL

INTERFACE

1nF

0.1

µF

0.1

µF

–2.5V

+2.5V

SINGLE-

ENDED

GROUND

REFERENCED

SOURCE

Figure 49. DC Coupling the AD8370. Dual supplies are used to set the input

and output common-mode levels to 0 V.

AD8370

AD8138

678

SERIAL CONTROL

INTERFACE

1nF

0.1

µF

+5V

499

Ω

499

Ω

100

Ω

499

Ω

499

Ω

100

Ω

VOCM

+5V

SINGLE-ENDED GROUND

REFERENCED SOURCE

Figure 50. DC Coupling the AD8370. The AD8138 is used as a unity gain level

shifting amplifier to lift the common-mode level of the source to midsupply.

The AD8370 is also a dc accurate variable gain amplifier. The

common-mode dc voltage present at the output pins is internally

set to midsupply using what is essentially a buffered resistive

divider network connected between the positive supply rail and

the common (ground) pins. The input pins are at a slightly

higher dc potential, typically 250 mV to 550 mV above the out-

put pins, depending on gain setting. In a typical single-supply

application, it is necessary to raise the common-mode reference

level of the source and load to roughly midsupply to maintain

symmetric swing and to avoid sinking or sourcing strong bias

currents from the input and output pins. It is possible to use

balanced dual supplies to allow ground referenced source and

load as indicated in Figure 49. By connecting the VOCM pin

and unused input to ground, the input and output common-

mode potentials are forced to virtual ground. This allows direct

coupling of ground referenced source and loads. The initial

differential input offset is typically only a few 100 µV. Over

temperature, the input offset could be as high as a few tens of

mVs. If precise dc accuracy is need over temperature and time, it

may be necessary to periodically measure the input offset and to

apply the necessary opposing offset to the unused differential

input, canceling the resulting output offset.

To address situations where dual supplies are not convenient, a

second option is presented in Figure 50. The AD8138 differential

amplifier is used to translate the common-mode level of the

driving source to midsupply, which allows dc accurate perform-

ance with a ground-referenced source without the need for dual

supplies. The bandwidth of the solution in Figure 50 is limited

by the gain-bandwidth product of the AD8138. The normalized

frequency response of both implementations is shown in Figure 51.

–10

–8

–6

–4

–2

100

10k

100k

10M

100M

FREQUENCY (Hz)

AD8370 WITH

AD8138 SINGLE

+5V SUPPLY

AD8370

USING DUAL

±2.5V SUPPLY

Figure 51. Normalized Frequency Response of the Two Solutions in

Figure 49 and Figure 50

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