AD694JNZ Datasheet AD694JNZ, AD694ARZ-REEL, AD694BRZ | P10-P12

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

10 V. Spans below 2 V require a V

of 4.5 V or greater.

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

voltage V

REF

= 2 V (R6 + R5)/R5. In choosing external

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

from BW ADJ (Pin 14) to V

(Pin 13) as shown in Figure 10.

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.

AD694

–10–

REV. B

Figure 11. Buffer Amplifier V

Adjustment

APPLICATIONS

CURRENT OUTPUT DAC INTERFACE

The AD694 can be easily interfaced to current output DACs

such as the AD566A to construct a digital to 4–20 mA interface

as shown in Figure 13. The AD694 provides the voltage refer-

ence and the buffer amplifier necessary to operate the DAC.

Only simple connections are necessary to construct the circuit.

The 10 V reference of the AD694 supplies reference input of the

AD566. The buffer amplifier converts the full-scale current to

+10 V utilizing the internal resistors in the DAC; therefore the

AD694 is configured for a 10 V full-scale input. A 10 pF capaci-

tor compensates for the 25 pF output capacitance of the DAC.

An optional 100 Ω trim resistor (R

) allows the full-scale to be

trimmed, a 50 Ω resistor may be substituted if a trim is not re-

quired; accuracy will be typically ±1 LSB and the trim does not

affect the 4 mA offset. Care should be taken in managing the

circuit grounds. Connections from AD694 Pins 9, 3 and AD566

Pins 3 and 7 should be as short as possible and to a single point

close to Pin 5 of the AD694. Best practice would have separate

connections to the star ground from each pin; this is essential

Figure 12. Using the Alarm to Drive a TTL Gate

ALARM CIRCUIT

The AD694 has an alarm circuit which warns of open circuit

conditions at I

OUT

(Pin 11), or of attempts to drive the voltage

at I

OUT

higher than V

– 2 V. The alarm transistor will pull

down if an out of control condition is sensed. The alarm current

is limited to about 20 mA.

Figure 12 shows a typical application. In a digital/analog system

the alarm can provide a TTL signal to a controller. The collec-

tor of the alarm transistor is tied to the system logic supply

through a 20 kΩ pull-up resistor. The alarm is off in normal op-

eration and the voltage at the alarm pin is high. In the event that

the wire from I

OUT

(Pin 11) is opened, or if a large input over-

drive forces I

OUT

too close to V

, then the alarm pin is driven

low. This configuration is compatible with CMOS or TTL logic

levels. The alarm transistor can also be used to directly drive an

LED or other indicators.

for the AD566 power ground from Pin 12. The 4–20 mA output

(Pin 11) must have a return path to the power ground. The re-

turn line from the load may be connected to the power ground,

or to the –15 V supply based upon the size of the load to be

driven, and on power dissipation considerations.

SINGLE-SUPPLY DIGITAL TO 4–20 mA INTERFACE

A 12 bit input to 4–20 mA output interface can be constructed

that operates on a single 15 V supply. The DAC is operated in

its voltage switching mode; this allows the DAC, when supplied

with a voltage reference of less than 2.5 V, to provide an output

voltage that is proportional to the digital input code and ranges

from 0 V to V

REF

. The AD694 voltage reference is connected to

supply 2 V and the input stage is set to a 2 V full scale; the input

buffer amplifier serves to buffer the voltage output from the

DAC. Connected in this manner, a full-scale DAC input code

will result in a 20 mA output and an all 0 code will result in a

4 mA output. The loading on the AD694 voltage reference is

AD694

REV. B

–11–

Figure 13. Digital to 4–20 mA Interface Using a Current Steering DAC

Figure 14. Single-Supply Digital Input to 4–20 mA Output

code dependent, and the response time of the circuit will be de-

termined by the reaction of the voltage reference. The supply

voltage to the AD7541A should be kept close to 15 V. If V

reduced significantly from 15 V the differential nonlinearity of

the DAC will increase and the linearity will be degraded.

In some applications it is desirable to have some underrange and

overrange in the 4–20 mA output. For example, assume an over

and under range capability of ±5% of span is needed, then the

output current range corresponding to the full scale of the DAC

is 3.2 mA to 20.8 mA. To accomplish this, the span of the

AD694 would be increased 10% to 17.6 mA by adding a nonin-

verting gain of 1.1 to the buffer amplifier. The 4 mA offset

would then be reduced by 0.8 mA, by utilizing the adjustment

scheme explained in Adjusting 4 mA Zero section. Then a digi-

tal input from all zero code to full scale would result in an out-

put current of 3.2 mA to 20.8 mA.

LOW COST SENSOR TRANSMITTER

Sensor bridges typically output differential signals in the 10 mV

to 100 mV full-scale range. With an AD694, a dual op amp, and

some resistors, an instrumentation amplifier front end can be

added which easily handles these types of low level signals.

The traditional 3 op amp instrumentation amplifier is built us-

ing an AD708 dual op amp for the front end, and the AD694’s

buffer amplifier is used for the subtractor circuit, as shown in

Figure 15. The AD694’s 2 V reference is used to provide a

“ground” of 2 V that ensures proper operation of the in amp

over a wide common mode range. The reference pin of the

subtractor circuit is tied to the 2 V reference (point C). A 2 kΩ

pull-down resistor ensures that the voltage reference will be able

to sink any subtractor current. The 2 V FS (Pin 4) is attached to

the 2 V reference; this offsets the input range of the V/I con-

verter 2 volts positive, to match the “ground” of the in amp.

The AD694 will now output a 4–20 mA output current for a

0 V to 2 V differential swing across V

. The gain of the in amp

front end is adjusted so that the desired full-scale input signal at

results in a V

of 2 V. For example a sensor that has a 100

mV full scale will require a gain of 20 in the front end. The gain

is determined according to the equation:

G = [2R

/Rg] + 1

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P17

AD694JNZ

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Description:

Instrumentation Amplifiers IC 4-20mA Mono Current Transmitter

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AD694JNZ

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