Circuit Note | Using the AD8599 Op Amp as an Ultralow Distortion Driver for the AD7995

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Using the AD8599 Op Amp as an Ultralow Distortion Driver for the AD7995

(CN0044)

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Circuit Types:	ADC Circuit/Driver
Optimized For:	High Speed, Low Noise and Distortion
Applications:	Building Controls, Communications, Instrumentation, Medical, Motion Control, Process Control

Circuit Function and Benefits

The circuit described in this document provides an ultralow distortion driver circuit for the AD7995 10-bit, 4-channel ADC, which is designed to achieve optimum ac and dc performance. The circuit uses the ultralow distortion, ultralow noise AD8599 dual-supply op amp and ultrahigh precision AD780 band gap voltage reference to ensure that the maximum AD7995 performance is achieved, by providing a low impedance driver with adequate settling time and a highly accurate reference voltage. The AD8599 is a dual operational amplifier that operates with supplies from ±4.5 V to ±18 V. The AD7995 has an I²C-compatible serial interface and is offered in an 8-lead SOT-23 package.

Figure 1: AD7995 ADC with the AD8599 Low Distortion Driver and AD780 Ultrahigh Precision Reference (Simplified Schematic: Decoupling and All Connections Not Shown)

Circuit Description

It is always recommended to buffer analog input signals before applying them to ADCs with switched capacitor inputs such as the AD7995. This is particularly important in applications where the signal source has high source impedance and where low distortion and high signal-to-noise-ratio is important. The circuit illustrated in Figure 1 shows how the AD8599, an ideal choice for high accuracy designs, can be used to buffer the analog input channels.

The AD7995 can be operated as a 4-channel input device using V_DD as a reference (the input voltage range is 0 V to V_DD) or as a 3-channel input device with the fourth channel used as an external reference input, V_REF (the input range is 0 V to V_REF). These options are programmable via the I²C-compatible interface.

The AD780 is a 2.5 V/3 V ultrahigh precision, band gap voltage reference and is recommended for use with AD7995. A 1 μF decoupling capacitor is recommended on the V_IN3 / V_REF signal for best performance.

Care must be taken to ensure that the analog input signal to the ADC does not exceed the supply rails by more than 300 mV. If the signal does exceed this level, the internal ESD protection diodes become forward-biased and start conducting current into the substrate. Each diode can conduct a maximum current of 10 mA without causing irreversible damage to the part. The MT-036 Tutorial discusses methods to protect the input circuits of op amps and ADCs against such damage.

In addition, the circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see MT-031 Tutorial, MT-101 Tutorial, and the AD7995 evaluation board layout).

Common Variations

If single-supply op amp operation is required for buffering the input signal, the AD8605 is a suitable choice. Note that the output of the AD8605 operating on a single +5 V supply can only go to approximately 20 mV above ground; therefore, the AD7995 input range from 0 V to +20 mV cannot be exercised (see MT-035 Tutorial).

The AD7995 can accept a reference input voltage from 1.2 V to V_DD; therefore, different voltage reference sources can be used.

Contributed October, 2008

Circuit Function and Benefits
Circuit Description
Common Variations

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AD780:

AD780: 2.5 V/3.0 V Ultrahigh Precision Bandgap Voltage Reference

The AD780 is an ultrahigh precision bandgap reference voltage which provides a 2.5 V or 3.0 V output from inputs between 4.0 V and 36 V. Low initial error and temperature drift combined with low output noise and the ability to drive any value of capacitance make the AD780 the ideal choice for enhancing the performance of high resolution ADCs and DACs and for any general purpose precision reference More

Data Sheet Rev E, 05/2004 (pdf 375kB)

2.5 V/3.0 V Ultrahigh Precision Bandgap Voltage Reference
Data Sheet Rev E, 05/2004 (pdf 375kB)
AD7995:

AD7995: 4-Channel, 10-Bit ADC with I²C Compatible Interface in 8-Lead SOT-23

The AD7991/AD7995/AD7999 are 12-/10-/8-bit, low power, successive approximation ADCs with an I²C®-compatible interface. Each part operates from a single 2.7 V to 5.5 V power supply and features a 1 μs conversion time. The track-and-hold amplifier allows each part to handle input frequencies of up to 14 MHz, and a multiplexer allows taking samples from four channels.

Each More

Data Sheet Rev A, 10/2009 (pdf 776kB)

4-Channel, 10-Bit ADC with I²C Compatible Interface in 8-Lead SOT-23
Data Sheet Rev A, 10/2009 (pdf 776kB)
AD8599:

AD8599: Dual, Ultralow Distortion, Ultralow Noise Op Amp

The AD8597/AD8599 are very low noise, low distortion opera-tional amplifiers ideal for use as preamplifiers. The low noise of 1.1 nV/√Hz and low harmonic distortion of −120 dB (or better) at audio bandwidths give the AD8597/AD8599 the wide dynamic range necessary for preamplifiers in audio, medical, and instru-mentation applications. The excellent slew rate of 14 V/μs and 10 MHz gain bandwidth More

Data Sheet Rev B, 11/2008 (pdf 534kB)

Dual, Ultralow Distortion, Ultralow Noise Op Amp
Data Sheet Rev B, 11/2008 (pdf 534kB)