AD694

REV. B

–9–

Figure 8. Span Adjustment, 2 V Full Scale

PROGRAMMING OTHER SPANS

There are two methods for programming input spans less than

10 V. The first decreases the input span by programming a non-

inverting gain into the buffer amplifier. For example, to achieve

an input span of 0–5 V, the AD694 is set in its 10 V full-scale

mode and the buffer amplifier is configured with a noninverting

gain of 2 by adding 2 resistors. Now a 5 V signal at +Sig results

in a 10 V full-scale signal at FB (Pin 1), the input to the V/I.

This method requires that the V/I be programmed to a 10 V full

scale for input spans between 2 V to 10 V. It should be pro-

grammed to a 2 V full scale if input spans of less than 2 V are

required. This adjustment scheme makes the accuracy of the

span adjustment dependent upon the ratio accuracy of the re-

quired gain resistors. Thus, it is possible to accurately configure

spans other than 2 V or 10 V without using trimming potenti-

ometers, given that the resistor ratios are sufficiently accurate. A

supply voltage of 12.5 V is required for spans between 2 V and

A second method, allows other spans of less than 10 V to be

programmed when supply voltage is less than 12.5 V. Since the

AD694 amplifiers require 2.5 V of headroom for operation, a

5 V full-scale input is possible with a 7.5 V supply. This is

achieved by placing a resistor, in parallel with R2, (2 V FS [Pin

4] to Com [Pin 5]), to adjust the transconductance of the V/I

converter without a headroom penalty. A disadvantage of this

method is that the external resistor must match the internal re-

sistor in a precise manner, thus a span trim will be required.

The value should be chosen to allow for the

±10% uncertainty

in the absolute value of the internal resistor R2.

ADJUSTING REFERENCE OUTPUT

Figure 9 shows one method of making small adjustments to the

10 V reference output. This circuit allows a linear adjustment

range of

±200 mV. The 2 V reference may also be adjusted but

only in the positive direction.

Other reference voltages can be programmed by adding external

resistors. For example, a resistor placed in parallel with R5 can

be added to boost the reference output as high as 20 V. Con-

versely, a resistor in parallel with R6 can be used to set the refer-

ence voltage to a value between 2 V and 10 V. The output

adjustment resistors remember that the internal resistors, while

ratio matched to a high degree of accuracy, have an absolute re-

sistor tolerance of only

±10%. Be prepared to compensate for

this if a precise voltage other than the precalibrated values of 2

V or 10 V is required.

Figure 9. 10 V Reference Output Adjustment

BANDWIDTH CONTROL

The bandwidth of the AD694 can be limited to provide noise

filtering. This is achieved by connecting an external capacitor

To program the bandwidth, substitute the desired bandwidth in

Hz, into the formula below to determine the required capacitor.

C

BW

=×

×

12

900

/(

)

πΩ

The bandwidth chosen will vary

±10% due to internal resistor

tolerance, plus an additional amount due to capacitor tolerance.

This method of bandwidth control is not recommended as a

way to filter large high frequency transients in the input signal.

It is recommended that frequencies greater than the BW of the

buffer amplifier be eliminated with an input filter to avoid recti-

fication of noise by the input amplifiers.

Figure 10. Noise Filtering with an External Capacitor

BUFFER AMPLIFIER OFFSET ADJUST

The buffer amplifier input voltage offset has been laser trimmed

to a high degree of accuracy; however, there may be occasions

when an offset trim is desired. Figure 11 shows the adjustment

method; a trim range of greater than

±2.5 mV is available with

this scheme. It is not recommended that this adjustment

method be used to affect the 4 mA offset current as the trim will

induce offset drift into the buffer amplifier. The buffer amplifier

will drift approximately 1

µV/°C for each 300 µV of induced

offset. To adjust the 4 mA offset current refer to the Adjusting

4 mA Zero section.