AD810 datasheet(16/22 Pages) AD

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AD810

Data Sheet

Rev. B | Page 16 of 22

OFFSET NULLING

A 10 kΩ potentiometer connected between Pin 1 and Pin 5,

with its wiper connected to +VS can be used to trim out the

inverting input current (with about ±20 µA of range). For

closed-loop gains above about 5, this configuration may not be

sufficient to trim the output offset voltage to zero. Tie the

potentiometer wiper to ground through a large value resistor

(50 kΩ for ±5 V supplies, 150 kΩ for ±15 V supplies) to trim

the output to zero at high closed-loop gains.

10kΩ

SEE TEXT

AD810

–VS

+VS

0.1µF

Figure 47. Offset Null Configuration

DISABLE MODE

By pulling the voltage on Pin 8 to ground (0 V), the AD810 can

be put into a disabled state. In this condition, the supply current

drops to less than 2.8 mA, the output becomes high impedance,

and there is a high level of isolation from input to output. In the

case of a line driver, for example, the output impedance is about

the same as for a 1.5 kΩ resistor (the feedback plus gain resistors)

in parallel with a 13 pF capacitor (due to the output) and the

input to output isolation is greater than 65 dB at 1 MHz.

Leaving the disable pin disconnected (floating) keeps the

AD810 operational in the enabled state.

The input impedance of the disable pin is about 35 kΩ in

parallel with a few pF. When grounded, about 50 µA flows out

of the DISABLE pin for ±5 V supplies. If driven by complementary

output CMOS logic, the disable time (until the output goes high

impedance) is about 100 ns and the enable time (to low impedance

output) is about 170 ns on ±5 V supplies. The enable time can

be extended to about 750 ns by using open-drain logic.

When operated on ±15 V supplies, the AD810 disable pin can

be driven by open-drain logic. In this case, adding a 10 kΩ

pull-up resistor from the DISABLE pin to the positive supply

decreases the enable time to about 150 ns. If there is a nonzero

voltage present on the amplifier output at the time the device is

switched to the disabled state, some additional decay time is

required for the output voltage to decrease to zero. The total

time for the output to go to zero is generally about 250 ns and is

somewhat dependent on the load impedance.

In cases where the amplifier is driving a high impedance load,

the input to output isolation decreases significantly if the input

signal is greater than about 1.2 V p-p. The isolation can be

restored back to the 65 dB level by adding an extra load (for

example, 150 Ω) at the amplifier output. This additional load

attenuates the feedthrough signal. (This decreased isolation is

not an issue for multiplexer applications where the outputs of

multiple AD810 devices are tied together as long as at least one

channel is in the on state.)

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