AD780

REV. B

–6–

TEMPERATURE OUTPUT PIN

The AD780 provides a “TEMP” output (Pin 3) that varies

linearly with temperature. This output can be used to monitor

changes in system ambient temperature and to initiate calibration

°C,

and the temperature coefficient is approximately 2 mV/

°C.

perature taken at the output of the op amp with a noninverting

gain of five.

4.25

2.00

150

2.50

2.25

–50

–75

2.75

3.00

3.25

3.50

3.75

4.00

125

100

75

50

25

0

–25

TEMPERATURE – C

CIRCUIT CALIBRATED AT 25 C

REFER TO FIGURE 9

10mV PER C

Figure 8. Temperature Pin Transfer Characteristic

Since the TEMP voltage is acquired from the bandgap core cir-

cuit, current pulled from this pin will have a significant effect on

suitable op amp, e.g., an OP07, AD820 or AD711 (all of which

would result in less than a 100

∆V

µA × I

or

∆V

µA × I

large amount of current, even in tens of microamp, drawn from

The choice of C1 and C2 was dictated primarily by the need for a

relatively flat response that rolled off early in the high frequency

noise at the output. But there is considerable margin in the

choice of these capacitors. For example, the user can actually

put a huge C2 on the TEMP pin with none on the output pin.

However, one must either put very little or a lot of capacitance at

the TEMP pin. Intermediate values of capacitance can sometimes

cause oscillation. In any case, the user should follow the recom-

mendation in Figure 3.

TEMPERATURE TRANSDUCER CIRCUIT

The circuit shown in Figure 9 is a temperature transducer which

a amplifies the TEMP output voltage by a gain of a little over 5

to provide a wider full scale output range. The trimpot can be

used to adjust the output so it varies exactly by 10 mV/

°C.

To minimize resistance changes with temperature, resistors with

low temperature coefficients, such as metal film resistors should

be used.

+5V

TEMP

GND

AD820

6.04k

(1%)

1.27k

(1%)

200

10mV/ C

1 F

AD780

Figure 9. Differential Temperature Transducer

SUPPLY CURRENT OVER TEMPERATURE

The AD780’s quiescent current will vary slightly over tempera-

ture and input supply range. The test limit is 1 mA over the in-

dustrial and 1.3 mA over the military temperature range.

Typical performance with input voltage and temperature varia-

tion is shown in Figure 10 following.

0.85

0.60

36

0.75

0.65

0.70

4

0.80

INPUT VOLTAGE – Volts

–55 C

125 C

25 C

Figure 10. Typical Supply Current over Temperature