AD780
LINE REGULATION
Line regulation is a measure of the change in output voltage due
to a specified change in input voltage. It is intended to simulate
worst case unregulated supply conditions and is measured in
µV/V.
Figure 14 shows typical performance with 4.0 V < V
IN
<
15.0 V.
200
T = 25 C
The AD780 is also ideal for use with higher resolution convert-
ers such as the AD7710/AD7711/AD7712. (See Figure 16.)
While these parts are specified with a 2.5 V internal reference,
the AD780 in 3 V mode can be used to improve the absolute ac-
curacy, temperature stability and dynamic range. It is shown fol-
lowing with the two optional noise reduction capacitors.
+5V
OUTPUT CHANGE – V
100
V
IN
0
1 F
V
OUT
REFIN+
AD780
100 F
–100
AD7710
100nF
GND
2.5/3.0V
SELECT
–200
4
10
INPUT VOLTAGE – Volts
15
REFIN–
Figure 14. Output Voltage Change vs. Input Voltage
PRECISION REFERENCE FOR HIGH RESOLUTION
+5 V DATA CONVERTERS
Figure 16. Precision 2.5 V or 3.0 V Reference for the
AD7710 High Resolution, Sigma-Delta ADC
+4.5 V REFERENCE FROM +5 V SUPPLY
The AD780 is ideally suited to be the reference for most +5 V
high resolution ADCs. The AD780 is stable under any capaci-
tive load, it has superior dynamic load performance, and the
3.0 V output provides the converter with maximum dynamic
range without requiring an additional and expensive buffer am-
plifier. One of the many ADCs that the AD780 is suited for is
the AD7884, a 16-bit, high speed sampling ADC. (See Figure
15.) This part previously needed a precision 5.0 V reference, re-
sistor divider and buffer amplifier to do this function.
+5V
Some +5 V high resolution ADCs can accommodate reference
voltages up to +4.5 V. The AD780 can be used to provide a
precision +4.5 V reference voltage from a +5 V supply using the
circuit shown following in Figure 17. This circuit will provide a
regulated +4.5 V output from a supply voltage as low as +4.7 V.
The high quality tantalum 10
µF
capacitor in parallel with the
ceramic 0.1
µF
capacitor and the 3.9
Ω
resistor ensure a low
output impedance up to around 50 MHz.
V
SUPPLY
0.1 F
1k
2N2907
V
IN
1 F
V
OUT
V
REF
+F
AD780
OP90
2.5k
10 F
V
OUT
0.1 F
AD780
V
REF
+S
GND
2.5/3.0V
SELECT
0.1 F
3.9
AD7884
5k
0.01%
4k
0.01%
Figure 17. +4.5 V Reference from a Single +5 V Supply
Figure 15. Precision 3.0 V Reference for the AD7884
16-Bit, High Speed ADC
–8–
REV. B
AD [ ANALOG DEVICES ]
