Datasheet

Data Sheet AD780
APPLYING THE AD780
analog.com Rev. J | 7 of 12
Figure 9. Noise Reduction Circuit
NOISE COMPARISON
The wideband noise performance of the AD780 can also be ex-
pressed in ppm. The typical performance with C1 and C2 is 0.6
ppm; without external capacitors, typical performance is 1.2 ppm.
This performance is, respectively, 7× and 3× lower than the speci-
fied performance of the LT1019.
Figure 10. Reduced Noise Performance with C1 = 100 µF, C2 = 100 nF
TEMPERATURE PERFORMANCE
The AD780 provides superior performance over temperature by
means of a combination of patented circuit design techniques, pre-
cision thin-film resistors, and drift trimming. Temperature perform-
ance is specified in terms of ppm/°C; because of nonlinearity in the
temperature characteristic, the box test method is used to test and
specify the part. The nonlinearity takes the form of the characteristic
S-shaped curve shown in Figure 11. The box test method forms
a rectangular box around this curve, enclosing the maximum and
minimum output voltages over the specified temperature range. The
specified drift is equal to the slope of the diagonal of this box.
Figure 11. Typical AD780BN Temperature Drift
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 of the system, if
desired. The voltage V
TEMP
is 560 mV at 25°C, and the temperature
coefficient is approximately 2 mV/°C.
Figure 12 shows the typical V
TEMP
characteristic curve over temper-
ature taken at the output of the op amp with a noninverting gain of
5.
Figure 12. Temperature Pin Transfer Characteristic
Since the TEMP voltage is acquired from the band gap core circuit,
current pulled from this pin has a significant effect on V
OUT
. Care
must be taken to buffer the TEMP output with a suitable op amp,
for example, an OP07, AD820, or AD711 (all of which would result
in less than a 100 µV change in V
OUT
). The relationship between
I
TEMP
and V
OUT
is
∆V
OUT
= 5.8 mV/µA I
TEMP
(2.5 V Range)
or
∆V
OUT
= 6.9 mV/µA I
TEMP
(3.0 V Range)