Datasheet AD7357 (Analog Devices) - 18

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Data Sheet AD7357 Operational Amplifier Pair VOLTAGE REFERENCE
An operational amplifier pair is used to directly couple a The AD7357 allows the choice of a very low temperature drift differential signal to one of the analog input pairs of the AD7357. internal voltage reference or an external reference. The internal The circuit configurations shown in Figure 22 and Figure 23 2.048 V reference of the AD7357 provides excellent performance show how an operational amplifier pair can be used to convert a and can be used in almost all applications. single-ended signal into a differential signal for a bipolar and When the internal reference is used, the reference voltage is unipolar input signal, respectively. The voltage applied to Point A present on the REFA and REFB pins. Decouple these pins to sets up the common-mode voltage. In both diagrams, Point A is REFGND with 10 μF capacitors. The internal reference voltage connected in some way to the reference. The AD8022 is a suitable can be used elsewhere in the system, provided it is buffered dual operational amplifier that is used in this configuration to externally. provide differential drive to the AD7357. The REF
2.048V
A and REFB pins can also be overdriven with an external
VREF p-p 220Ω 1.024V
voltage reference if desired. The applied reference voltage can
VREF 220Ω V+ 0V 2 27Ω
range from 2.048 V + 100 mV to VDD. A common choice is to
V GND IN+
use an external 2.5 V reference such as the ADR441 or ADR431.
V– 220Ω ADC TRANSFER FUNCTION 2.048V AD7357* 220Ω 1.024V
The output coding for the AD7357 is straight binary. The designed
V+ 0V 27Ω
code transitions occur at successive LSB values (such as 1 LSB
V REFA/REFB IN– A
or 2 LSB). The LSB size is (2 × VREF)/16,384. The ideal transfer
V– 10kΩ
characteristic of the AD7357 is shown in Figure 24.
10µF 10kΩ
32 1
111 ... 111
757-
*ADDITIONAL PINS OMITTED FOR CLARITY.
07
111 ... 110 111 ... 101
Figure 22. Dual Operational Amplifier Circuit to Convert a Single-Ended Unipolar Signal into a Differential Signal
E D 2.048V 2 × VREF p-p 220Ω 1.024V 440Ω V+ 0V DC CO GND A 27Ω VIN+ V– 000 ... 010 220Ω 2.048V AD7357* 000 ... 001 220Ω 220Ω 1.024V 000 ... 000 V+ 0V –VREF + 1 LSB +VREF – 1 LSB 27Ω
3
V REF –V +V A/REFB
02
IN– REF + 0.5 LSB REF – 1.5 LSB A
7-
ANALOG INPUT
75
V–
07
20kΩ
Figure 24. Deal Transfer Characteristic
10µF 10kΩ
3 13 57-
*ADDITIONAL PINS OMITTED FOR CLARITY.
077 Figure 23. Dual Operational Amplifier Circuit to Convert a Single-Ended Bipolar Signal into a Differential Unipolar Signal Rev. E | Page 17 of 24 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS REVISION HISTORY SPECIFICATIONS TIMING SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS TERMINOLOGY THEORY OF OPERATION CIRCUIT INFORMATION CONVERTER OPERATION ANALOG INPUT STRUCTURE ANALOG INPUTS DRIVING DIFFERENTIAL INPUTS Differential Amplifier Operational Amplifier Pair VOLTAGE REFERENCE ADC TRANSFER FUNCTION MODES OF OPERATION NORMAL MODE PARTIAL POWER-DOWN MODE FULL POWER-DOWN MODE POWER-UP TIMES POWER vs. THROUGHPUT RATE SERIAL INTERFACE APPLICATION SUGGESTIONS GROUNDING AND LAYOUT EVALUATING THE AD7357 PERFORMANCE OUTLINE DIMENSIONS ORDERING GUIDE AUTOMOTIVE PRODUCTS