AD780 Data Sheet
Rev. H | Page 10 of 12
PRECISION REFERENCE FOR HIGH RESOLUTION
5 V DATA CONVERTERS
The AD780 is ideally suited to be the reference for most 5 V
high resolution ADCs. The AD780 is stable under any capacitive
load, has superior dynamic load performance, and its 3.0 V
output provides the converter with the maximum dynamic
range without requiring an additional and expensive buffer
amplifier. One of the many ADCs that the AD780 is suited for is
the AD7884, a 16-bit, high speed sampling ADC (see Figure 21).
This part previously needed a precision 5 V reference, resistor
divider, and buffer amplifier to do this function.
00841-021
+V
IN
AD780
AD7884
V
OUT
V
REF
+ F
6
V
REF
+ S
2.5V/3.0V
SELECT
8
GND
4
2
1µF
5V
Figure 21. Precision 3 V Reference for the AD7884 16-Bit, High Speed ADC
The AD780 is also ideal for use with higher resolution converters,
such as the AD7710/AD7711/AD7712 (see Figure 22. 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 accuracy,
temperature stability, and dynamic range. It is shown in Figure 22
with the two optional noise reduction capacitors.
00841-022
+V
IN
AD780
AD7710
V
OUT
REF IN+
6
2.5V/3.0V
O/P SELECT
8
GND
4
3
2
1µF
100nF
5V
REF IN–
100µF
Figure 22. Precision 2.5 V or 3.0 V Reference for the
AD7710 High Resolution, Σ-Δ ADC
4.5 V REFERENCE FROM 5 V SUPPLY
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 in Figure 23. This circuit provides 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
AD780 0.1 µF capacitor and the 3.9 Ω resistor, ensures a low
output impedance around 50 MHz.
00841-023
AD780
2N2907
V
OUT
5kΩ
0.01%
4kΩ
0.01%
2.5kΩ
3.9Ω
1kΩ
0.1µF
0.1µF
10µF
0.1µF
OP90
+
–
V
SUPPLY
2
4
6
3
7
6
4
2
Figure 23. 4.5 V Reference from a Single 5 V Supply
NEGATIVE (–2.5 V) REFERENCE
The AD780 can produce a negative output voltage in shunt mode
by connecting the input and output to ground, and connecting
the GND pin of the AD780 to a negative supply via a bias resistor,
as shown in Figure 25.
00841-024
DNC
TEMP
1µF
+V
IN
V
OUT
TRIM
GND
O/P SELECT
2.5V – DNC
3.0V – GND
DNC
AD780
NOTES
1. I
L
= LOAD CURRENT
2. I
S
MIN = MINIMUM SHUNT CURRENT
3. DNC = DO NOT CONNECT TO THIS PIN
1
7
6
5
84
2
3
R =
V
OUT
– (V–)
I
L
+ I
S
MIN
–2.5 V
OUT
V–
Figure 24. Negative (−2.5 V Shunt Mode Reference)
A precise –2.5 V reference capable of supplying up to 100 mA to
a load can be implemented with the AD780 in series mode, using
the bootstrap circuit shown in Figure 25.
00841-025
AD780
2N3906
+5V
+5V
OP07
CONNECT IF
–3V OUTPUT
DESIRED
–2.5V (I
L
≤ 100mA)
–5V
–5V
OUT
1kΩ
1000pF
+V
IN
–
+
2
4
6 8
Figure 25. −2.5 V High Load Current Reference
