Datasheet AD780 (Analog Devices) - 10

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AD780 Data Sheet PRECISION REFERENCE FOR HIGH RESOLUTION VSUPPLY 5 V DATA CONVERTERS 0.1

F
The AD780 is ideally suited to be the reference for most 5 V
1k
high resolution ADCs. The AD780 is stable under any capacitive
2N2907 2
load, has superior dynamic load performance, and its 3.0 V
7 6 3 +
output provides the converter with the maximum dynamic
AD780 OP90 6 VOUT 2 – 2.5k
range without requiring an additional and expensive buffer
4 10

F 0.1

F
amplifier. One of the many ADCs that the AD780 is suited for is
4 0.1

F
the AD7884, a 16-bit, high speed sampling ADC (see Figure 21).
3.9
This part previously needed a precision 5 V reference, resistor divider, and buffer amplifier to do this function.
5k 4k 0.01% 0.01% 5V
00841-023 Figure 23. 4.5 V Reference from a Single 5 V Supply
AD7884 2 NEGATIVE (–2.5 V) REFERENCE +VIN
The AD780 can produce a negative output voltage in shunt mode
VOUT 6 VREF + F 1

F
by connecting the input and output to ground, and connecting
AD780
the GND pin of the AD780 to a negative supply via a bias resistor,
VREF + S
as shown in Figure 25.
2.5V/3.0V GND SELECT 4 8
021
2 7 +VIN DNC
00841- Figure 21. Precision 3 V Reference for the AD7884 16-Bit, High Speed ADC
VOUT 6 1 DNC
The AD780 is also ideal for use with higher resolution converters,
AD780 1µF
such as the AD7710/AD7711/AD7712 (see Figure 22). While these
TRIM 5
parts are specified with a 2.5 V internal reference, the AD780 in
3 TEMP
3 V mode can be used to improve the absolute accuracy,
O/P SELECT 2.5V – DNC
temperature stability, and dynamic range. It is shown in Figure 22
GND 3.0V – GND 4 8
with the two optional noise reduction capacitors.
–2.5 VOUT 5V VOUT – (V–) NOTES R = IL + IS MIN 1. IL = LOAD CURRENT AD7710 2. I
024
S MIN = MINIMUM SHUNT CURRENT 3. DNC = DO NOT CONNECT TO THIS PIN 2 V–
0841- 0
+VIN
Figure 24. Negative (−2.5 V Shunt Mode Reference)
VOUT 6 REF IN+
A precise –2.5 V reference capable of supplying up to 100 mA to
1

F AD780
a load can be implemented with the AD780 in series mode, using
3 100

F
the bootstrap circuit shown in Figure 25.
2.5V/3.0V 100nF GND O/P SELECT +5V 4 8 +V
022
IN REF IN– 2
00841- Figure 22. Precision 2.5 V or 3.0 V Reference for the
1k OUT AD780 6 8
AD7710 High Resolution, Σ-Δ ADC
+5V 4.5 V REFERENCE FROM 5 V SUPPLY CONNECT IF –3V OUTPUT 4 DESIRED
Some 5 V high resolution ADCs can accommodate reference
– –2.5V (I

L 100mA)
voltages up to 4.5 V. The AD780 can be used to provide a
OP07 2N3906
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
–5V –5V
quality tantalum 10 μF capacitor, in parallel with the ceramic AD780 0.1 μF capacitor and the 3.9 Ω resistor, ensures a low
1000pF
0841-025 0 output impedance around 50 MHz. Figure 25. −2.5 V High Load Current Reference Rev. I | Page 10 of 12 Document Outline Features Functional Block Diagram General Description Product Highlights Table of Contents Revision History Specifications Absolute Maximum Ratings Thermal Resistance Notes ESD Caution Theory of Operation Applying the AD780 Noise Performance Noise Comparison Temperature Performance Temperature Output Pin Temperature Transducer Circuit Supply Current Over Temperature Turn-On Time Dynamic Performance Line Regulation Precision Reference for High Resolution 5 V Data Converters 4.5 V Reference from 5 V Supply Negative (–2.5 V) Reference Outline Dimensions Ordering Guide