Datasheet

Data Sheet AD780
APPLYING THE AD780
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The AD780 can be used without any external components to
achieve specified performance. If power is supplied to Pin 2 and Pin
4 is grounded, Pin 6 provides a 2.5 V or 3.0 V output depending on
whether Pin 8 is left unconnected or grounded.
A bypass capacitor of 1 µF (+V
IN
to GND) should be used if the
load capacitance in the application is expected to be greater than 1
nF. The AD780 in 2.5 V mode typically draws 700 µA of I
q
at 5 V.
This increases by ~2 µA/V up to 36 V.
Figure 5. Optional Fine-Trim Circuit
Initial error can be nulled using a single 25 kΩ potentiometer
connected between V
OUT
, TRIM, and GND. This is a coarse trim
with an adjustment range of 4%, and is only included here for
compatibility purposes with other references. A fine trim can be
implemented by inserting a large value resistor (e.g., 1 MΩ to 5
MΩ) in series with the wiper of the potentiometer (see Figure 5).
The trim range, expressed as a fraction of the output, is simply
greater than or equal to 2.1 kΩ/R
NULL
for either the 2.5 V or 3.0 V
mode.
The external null resistor affects the overall temperature coefficient
by a factor equal to the percentage of V
OUT
nulled.
For example, a 1 mV (0.03%) shift in the output caused by the trim
circuit, with a 100 ppm/°C null resistor, adds less than 0.06 ppm/°C
to the output drift (0.03% × 200 ppm/°C, since the resistors internal
to the AD780 also have temperature coefficients of less than 100
ppm/°C).
NOISE PERFORMANCE
The impressive noise performance of the AD780 can be further
improved, if desired, by adding two capacitors: a load capacitor
(C1) between the output and ground, and a compensation capacitor
(C2) between the TEMP pin and ground. Suitable values are shown
in Figure 6.
Figure 6. Compensation and Load Capacitor Combinations
C1 and C2 also improve the settling performance of the AD780
when subjected to load transients. The improvement in noise per-
formance is shown in Figure 7, Figure 8, Figure 9, and Figure 10.
Figure 7. Standalone Noise Performance
Figure 8. Standalone Noise Performance