AD630 datasheet(14/20 Pages) AD

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AD630

Data Sheet

Rev. G | Page 14 of 20

OTHER GAIN CONFIGURATIONS

Many applications require switched gains other than the ±1 and

±2, which the self-contained applications resistors provide. The

AD630 can be readily programmed with three external resistors

over a wide range of positive and negative gain by selecting and

RB and RF to give the noninverting gain 1 + RF/RB and subsequent

RA to give the desired inverting gain. Note that when the

inverting magnitude equals the noninverting magnitude, the

value of RA is found to be RBRF/(RB + RF). That is, RA equals

the parallel combination of RB and RF to match positive and

negative gain.

The feedback synthesis of the AD630 may also include reactive

impedance. The gain magnitudes match at all frequencies if the

A impedance is made to equal the parallel combination of the

B and F impedances. The same considerations apply to the

AD630 as to conventional op amp feedback circuits. Virtually

any function that can be realized with simple noninverting L

network feedback can be used with the AD630. A common

arrangement is shown in Figure 25. The low frequency gain of

this circuit is 10. The response has a pole (−3 dB) at a frequency

≃1/(2π100kΩ×C)andazero(3dBfromthehighfrequency

asymptote) at about 10 times this frequency. The 2 kΩ resistor

in series with each capacitor mitigates the loading effect on

circuitry driving this circuit, eliminates stability problems, and

has a minor effect on the pole-zero locations.

As a result of the reactive feedback, the high frequency

components of the switched input signal are transmitted at

unity gain while the low frequency components are amplified.

This arrangement is useful in demodulators and lock-in amplifiers.

It increases the circuit dynamic range when the modulation or

interference is substantially larger than the desired signal

amplitude. The output signal contains the desired signal multiplied

by the low frequency gain (which may be several hundred for

large feedback ratios) with the switching signal and interference

superimposed at unity gain.

Figure 25. AD630 with External Feedback

SWITCHED INPUT IMPEDANCE

The noninverting mode of operation is a high input impedance

configuration while the inverting mode is a low input impedance

configuration. This means that the input impedance of the

circuit undergoes an abrupt change as the gain is switched

under control of the comparator. If the gain is switched when

the input signal is not zero, as it is in many practical cases, a

transient is delivered to the circuitry driving the AD630. In

most applications, this requires the AD630 circuit to be driven

by a low impedance source, which remains stiff at high frequencies.

This is generally a wideband buffer amplifier.

FREQUENCY COMPENSATION

The AD630 combines the convenience of internal frequency

compensation with the flexibility of external compensation by

means of an optional self-contained compensation capacitor.

In gain of ±2 applications, the noise gain that must be addressed

for stability purposes is actually 4. In this circumstance, the

phase margin of the loop is on the order of 60° without the

optional compensation. This condition provides the maximum

bandwidth and slew rate for closed loop gains of |2| and above.

When the AD630 is used as a multiplexer, or in other

configurations where one or both inputs are connected for

unity gain feedback, the phase margin is reduced to less than

20°. This may be acceptable in applications where fast slewing

is a first priority, but the transient response is not optimum. For

these applications, the self-contained compensation capacitor

may be added by connecting Pin 12 to Pin 13. This connection

reduces the closed-loop bandwidth somewhat and improves the

phase margin.

For intermediate conditions, such as a gain of ±1 where the loop

attenuation is 2, determine the use of the compensation by whether

bandwidth or settling response must be optimized. Also, use

optional compensation when the AD630 is driving capacitive

loads or whenever conservative frequency compensation is

desired.

–VS

10kΩ

11.11kΩ

100kΩ

2kΩ

SEL B

SEL A

CHANNEL

STATUS

B/A

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