AD8370 datasheet(18/28 Pages) AD

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AD8370

Rev. A | Page 18 of 28

If the amplifier is driven in single-ended mode, the input

impedance varies depending on the value of the resistor used to

terminate the other input as

RinSE = RinDIFF + RTERM

where RTERM is the termination resistor connected to the other

input.

–100

–90

–80

–70

–60

–50

–40

326496

128

GAIN CODE

HD2

LOW GAIN MODE

HIGH GAIN MODE

HD3

Figure 50. Harmonic Distortion of the Circuit in Figure 47

DC-COUPLED OPERATION

AD8370

SERIAL CONTROL

INTERFACE

1nF

0.1

μF

0.1

μF

–2.5V

+2.5V

SINGLE-

ENDED

GROUND

REFERENCED

SOURCE

Figure 51. DC Coupling the AD8370. Dual supplies are used to set the input

and output common-mode levels to 0 V.

AD8370

AD8138

SERIAL CONTROL

INTERFACE

1nF

0.1

μF

+5V

499

100

499

100

VOCM

+5V

SINGLE-ENDED GROUND

REFERENCED SOURCE

Figure 52. 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 VGA. 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 output 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 shown in Figure 51. 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 needed 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 52. The AD8138 differential

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

driving source to midsupply, which allows dc accurate

performance with a ground-referenced source without the need

for dual supplies. The bandwidth of the solution in Figure 52 is

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

normalized frequency response of both implementations is shown

in Figure 53.

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