Datasheet

Data Sheet AD780
APPLYING THE AD780
analog.com Rev. J | 8 of 12
Notice how sensitive the current dependent factor on V
OUT
is. A
large amount of current, even in tens of microamps, drawn from the
TEMP pin can cause the V
OUT
and TEMP output to fail.
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. However, 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 oscil-
lation. In any case, the user should follow the recommendation in
Figure 6.
TEMPERATURE TRANSDUCER CIRCUIT
The circuit shown in Figure 13 is a temperature transducer that
amplifies the TEMP output voltage by a gain of a little over +5 to
provide a wider full-scale output range. The digital potentiometer
can be used to adjust the output so it varies by exactly 10 mV/°C.
To minimize resistance changes with temperature, resistors with
low temperature coefficients, such as metal film resistors, should be
used.
Figure 13. Differential Temperature Transducer
SUPPLY CURRENT OVER TEMPERATURE
The quiescent current of the AD780 varies slightly over temperature
and input supply range. The test limit is 1 mA over the industrial
and 1.3 mA over the military temperature range. Typical perform-
ance with input voltage and temperature variation is shown in
Figure 14.
Figure 14. Typical Supply Current over Temperature
TURN-ON TIME
The time required for the output voltage to reach its final value
within a specified error band is defined as the turn-on settling time.
The two major factors that affect this are the active circuit settling
time and the time for the thermal gradients on the chip to stabilize.
Typical settling performance is shown in Figure 15. The AD780
settles to within 0.1% of its final value within 10 µs.
Figure 15. Turn-On Settling Time Performance
DYNAMIC PERFORMANCE
The output stage of the AD780 has been designed to provide
superior static and dynamic load regulation.
Figure 16 and Figure 17 show the performance of the AD780 while
driving a 0 mA to 10 mA load.