Circuit Note | AD7266 SAR ADC in Differential and Single-Ended Applications

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AD7266 SAR ADC in Differential and Single-Ended Applications

(CN0039)

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Circuit Types:	ADC Circuit/Driver
Optimized For:	High Performance, High Precision
Applications:	Instrumentation, Process Control

Circuit Function and Benefits

The driver circuits shown here are optimized for DC coupling applications requiring best distortion performance. This drive circuit insures that the maximum AD7266 performance is achieved, by providing adequate settling time and low impedance.

Figure 1: AD8022 Circuit to Convert a Single-Ended Unipolar Signal into a Differential Signal

Circuit Description

In applications where the signal source has high impedance, it is recommended to buffer the analog input signal before applying it to the inputs of the AD7266. A dual op-amp pair can be used to directly couple a differential signal to one of the analog input pairs of the AD7266. The AD8022 is an ideal choice of dual op-amp which can be used to provide a differential driver for the AD7266. The AD8022 is a dual, low power amplifier with low noise and low distortion. The AD7266 has a specified minimum acquisition time of 90ns with a VDD of 5V; this is the time from when the part enters track mode (13th SCLK rising edge) until the next conversion is initiated (CS falling edge). The op-amp selected for use with the AD7266 must have adequate settling time to meet the acquisition time requirements of the AD7266 and achieve the specified performance.

The circuit configurations illustrated in Figure 1 shows how an AD8022 op-amp can be used to convert a bipolar single-ended signal into a unipolar differential signal which can be applied directly to the AD7266 analog inputs. The voltage applied to Point A sets up the common-mode voltage. In Figure 1, point A is connected to the reference, but any value of specified common-mode range for the AD7266 can be used here to set up the desired common mode. If the on-chip 2.5 V reference on the AD7266 is to be used elsewhere in a system, (as illustrated in Figure1 and Figure 2) the output from D_CAPA and D_CAPB must first be buffered. The OP177 features the highest precision performance of any op-amp currently available, and is a perfect choice for a reference buffer.

Figure 2: AD8022 Circuit for Single-Ended Mode of Operation

The AD7266 can have a total of 12 single-ended analog input channels. The analog input range can be programmed to be either 0 to V_REF or 0 to 2 × V_REF. Figure 2 shows a typical connection diagram when operating the ADC in single-ended mode, where an AD8022 is used to drive a pair of discrete channels. The AD8021 is a high performance single op-amp which could be used as an alternative to a dual device, in very high performance systems.

Common Variations

The OP07D, an ultra-low offset voltage op-amp, is a lower cost alternative to the OP177. It offers similar performance with the exception of the Vos specification.

Contributed November, 2008

Circuit Function and Benefits
Circuit Description
Common Variations

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AD7266:

AD7266: Differential/Single-Ended Input, Dual, Simultaneous Sampling, 2 MSPS, 12-Bit, 3-Channel SAR A/D Converter

The AD7266* is a dual, 12-bit, high speed, low power, successive approximation ADC that operates from a single 2.7 V to 5.25 V power supply and features throughput rates up to 2 MSPS. The device contains two ADCs, each preceded by a 3-channel multiplexer, and a low noise, wide bandwidth track-and-hold amplifier that can handle input frequencies in excess of 30 MHz.
The conversion process More

Data Sheet Rev A, 12/2006 (pdf 682kB)

Differential/Single-Ended Input, Dual, Simultaneous Sampling, 2 MSPS, 12-Bit, 3-Channel SAR A/D Converter
Data Sheet Rev A, 12/2006 (pdf 682kB)
AD8022:

AD8022: Dual High Speed, Low Noise Op Amp

The AD8022 consists of two low noise, high speed, voltage feedback amplifiers. Each amplifier consumes only 4.0 mA of quiescent current, yet has only 2.5 nV/√Hz of voltage noise. These dual amplifiers provide wideband, low distortion performance, with high output current optimized for stability when driving capacitive loads. Manufactured on ADI's high voltage generation of XFCB bipolar More

Data Sheet Rev B, 05/2005 (pdf 364kB)

Dual High Speed, Low Noise Op Amp
Data Sheet Rev B, 05/2005 (pdf 364kB)
OP177:

OP177: Ultra-Precision Operational Amplifier

The OP177 features one of the highest precision performance of any op amp currently available. Offset voltage of the OP177 is only 25 μV maximum at room temperature. The ultralow VOS of the OP177 combines with its exceptional offset voltage drift (TCVOS) of 0.1 μV/°C maximum to eliminate the need for external VOS adjustment and increases system accuracy over temperature.

The OP177 More

Data Sheet Rev F, 03/2009 (pdf 340kB)

Ultra-Precision Operational Amplifier
Data Sheet Rev F, 03/2009 (pdf 340kB)