AD780
Data Sheet
V
5V
2
SUPPLY
AD7884
0.1F
1k
+V
IN
2N2907
2
V
6
V
V
+ F
+ S
OUT
REF
REF
7
+
3
6
1F
6
OP90
AD780
AD780
V
OUT
2.5k
2
–
4
10F
0.1F
2.5V/3.0V
SELECT
4
GND
4
0.1F
3.9
8
4k
0.01%
5k
0.01%
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.
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 AD780’s GND pin to a negative supply via a bias
resistor, as shown in Figure 25.
5V
2
7
AD7710
NC
+V
IN
2
+V
IN
V
6
5
OUT
1
3
NC
V
6
REF IN+
AD780
OUT
1F
1F
TRIM
AD780
TEMP
3
100F
O/P SELECT
2.5V – NC
3.0V – GND
2.5V/3.0V
GND O/P SELECT
100nF
GND
4
4
8
8
REF IN–
–2.5 V
OUT
V
– (V–)
OUT
+ I MIN
NOTES
R =
I
1. I = LOAD CURRENT
L
S
L
Figure 22. Precision 2.5 V or 3.0 V Reference for the
AD7710 High Resolution, Σ-Δ ADC
2. I MIN = MINIMUM SHUNT CURRENT
3. NC = NO CONNECT
S
V–
4.5 V REFERENCE FROM 5 V SUPPLY
Figure 24. Negative (−2.5 V Shunt Mode Reference)
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.
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.
+5V
+V
IN
2
OUT
1k
+5V
AD780
6
8
CONNECT IF
–3V OUTPUT
DESIRED
4
–
+
–2.5V (IL 100mA)
2N3906
OP07
–5V
–5V
1000pF
Figure 25. −2.5 V High Load Current Reference
Rev. F | Page 10 of 12
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