AD534JDZ datasheet(13/21 Pages) AD

13 / 21 page

AD534

Rev. C | Page 12 of 20

FUNCTIONAL DESCRIPTION

Figure 1 shows a functional block diagram of the AD534. Inputs

are converted to differential currents by three identical voltage-

to-current converters, each trimmed for zero offset. The product

of the X and Y currents is generated by a multiplier cell using

Gilbert’s translinear technique. An on-chip buried Zener

provides a highly stable reference, which is laser trimmed to

provide an overall scale factor of 10 V. The difference between

XY/SF and Z is then applied to the high gain output amplifier.

This permits various closed-loop configurations and dramati-

cally reduces nonlinearities due to the input amplifiers, a

dominant source of distortion in earlier designs.

The effectiveness of the new scheme can be judged from the

fact that, under typical conditions as a multiplier, the nonlinear-

ity on the Y input, with X at full scale (±10 V), is ±0.005% of FS.

Even at its worst point, which occurs when X = ±6.4 V, nonlinear-

ity is typically only ±0.05% of FS. Nonlinearity for signals applied

to the X input, on the other hand, is determined almost entirely

by the multiplier element and is parabolic in form. This error is a

major factor in determining the overall accuracy of the unit and

therefore is closely related to the device grade.

The generalized transfer function for the AD534 is given by

()(

) ()

OUT

−

where:

A is the open-loop gain of the output amplifier, typically

70 dB at dc.

X1, Y1, Z1, X2, Y2, and Z2 are the input voltages (full scale = ±SF,

peak = ±1.25 SF).

SF is the scale factor, pretrimmed to 10.00 V but adjustable by

the user down to 3 V.

In most cases, the open-loop gain can be regarded as infinite,

and SF is 10 V. The operation performed by the AD534, can

then be described in terms of the following equation:

(X1 − X2)(Y1 −Y2 ) = 10 V (Z1 − Z2)

The user can adjust SF for values between 10.00 V and 3 V by

connecting an external resistor in series with a potentiometer

between SF and −VS. The approximate value of the total

resistance for a given value of SF is given by the relationship:

−

kΩ

Due to device tolerances, allowance should be made to vary RSF

by ±25% using the potentiometer. Considerable reduction in

bias currents, noise, and drift can be achieved by decreasing SF.

This has the overall effect of increasing signal gain without the

customary increase in noise. Note that the peak input signal is

always limited to 1.25 SF (that is, ±5 V for SF = 4 V) so the

overall transfer function shows a maximum gain of 1.25. The

performance with small input signals, however, is improved by

using a lower scale factor because the dynamic range of the

inputs is now fully utilized. Bandwidth is unaffected by the use

of this option.

Supply voltages of ±15 V are generally assumed. However,

satisfactory operation is possible down to ±8 V (see Figure 7).

Because all inputs maintain a constant peak input capability of

±1.25 SF, some feedback attenuation is necessary to achieve

output voltage swings in excess of ±12 V when using higher

supply voltages.

PROVIDES GAIN WITH LOW NOISE

The AD534 is the first general-purpose multiplier capable of

providing gains up to ×100, frequently eliminating the need for

separate instrumentation amplifiers to precondition the inputs.

The AD534 can be very effectively employed as a variable gain

differential input amplifier with high common-mode rejection.

The gain option is available in all modes and simplifies the

implementation of many function-fitting algorithms such as

those used to generate sine and tangent. The utility of this

feature is enhanced by the inherent low noise of the AD534:

90 μV rms (depending on the gain), a factor of 10 lower than

previous monolithic multipliers. Drift and feedthrough are also

substantially reduced over earlier designs.

OPERATION AS A MULTIPLIER

Figure 15 shows the basic connection for multiplication. Note

that the circuit meets all specifications without trimming.

AD534

+VS

OUT

–VS

+15V

–15V

X INPUT

±10V FS

±12V PK

Y INPUT

±10V FS

±12V PK

OPTIONAL SUMMING

INPUT, Z, ±10V PK

OUTPUT, ±12V PK =

(X1 – X2) (Y1 – Y2)

10V

+ Z2

Figure 15. Basic Multiplier Connection

To reduce ac feedthrough to a minimum (as in a suppressed

carrier modulator), apply an external trim voltage (±30 mV

range required) to the X or Y input (see Figure 3). Figure 10

shows the typical ac feedthrough with this adjustment mode.

Note that the Y input is a factor of 10 lower than the X input

and should be used in applications where null suppression is

critical.

The high impedance Z2 terminal of the AD534 can be used to

sum an additional signal into the output. In this mode, the

output amplifier behaves as a voltage follower with a 1 MHz

small signal bandwidth and a 20 V/μs slew rate. This terminal

should always be referenced to the ground point of the driven

system, particularly if this is remote. Likewise, the differential

inputs should be referenced to their respective ground poten-

tials to realize the full accuracy of the AD534.

A much lower scaling voltage can be achieved without any

reduction of input signal range using a feedback attenuator as

shown in Figure 16. In this example, the scale is such that VOUT =

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