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AD7651

Driver Amplifier Choice

TYPICAL CONNECTION DIAGRAM

Although the AD7651 is easy to drive, the driver amplifier needs

to meet the following requirements:

Figure 22 shows a typical connection diagram for the AD7651.

Analog Input

•

The driver amplifier and the AD7651 analog input circuit

must be able to settle for a full-scale step of the capacitor

array at a 16-bit level (0.0015%). In the amplifier’s data

sheet, settling at 0.1% to 0.01% is more commonly speci-

fied. This could differ significantly from the settling time at

a 16-bit level and should be verified prior to driver

selection. The tiny op amp OP184, which combines ultra

low noise and high gain-bandwidth, meets this settling

time requirement.

Figure 23

Figure 23. Equivalent Analog Input Circuit

shows an equivalent circuit of the input structure of

the AD7651.

The two diodes, D1 and D2, provide ESD protection for the

analog inputs IN and INGND. Care must be taken to ensure

that the analog input signal never exceeds the supply rails by

more than 0.3 V. This will cause these diodes to become

forward-biased and start conducting current. These diodes can

handle a forward-biased current of 100 mA maximum. For

instance, these conditions could eventually occur when the

input buffer’s (U1) supplies are different from AVDD. In such a

case, an input buffer with a short-circuit current limitation can

be used to protect the part.

•

The noise generated by the driver amplifier needs to be

kept as low as possible in order to preserve the SNR and

transition noise performance of the AD7651. The noise

coming from the driver is filtered by the AD7651 analog

input circuit 1-pole low-pass filter made by R1 and C2 or

by the external filter, if one is used.

OR INGND

AGND

AVDD

02965-0-008

•

The driver needs to have a THD performance suitable to

that of the AD7651.

The OP184, OP162 or AD8519 meet these requirements and are

usually appropriate for almost all applications. As an alternative,

in very high speed and noise-sensitive applications, the AD8021

with an external 10 pF compensation capacitor can be used.

This capacitor should have good linearity as an NPO ceramic or

mica type. Moreover, the use of a noninverting +1 gain

arrangement is recommended and helps to obtain the best

signal-to-noise ratio.

This analog input structure allows the sampling of the

differential signal between IN and INGND. Unlike other

converters, INGND is sampled at the same time as IN. By using

this differential input, small signals common to both inputs are

rejected. For instance, by using INGND to sense a remote signal

ground, ground potential differences between the sensor and

the local ADC ground are eliminated.

The AD8022 could also be used if a dual version is needed and

gain of 1 is present. The AD829 is an alternative in applications

where high frequency (above 100 kHz) performance is not

required. In gain of 1 applications, it requires an 82 pF

compensation capacitor. The AD8610 is an option when low

bias current is needed in low frequency applications.

During the acquisition phase, the impedance of the analog input

IN can be modeled as a parallel combination of capacitor C1

and the network formed by the series connection of R1 and C2.

C1 is primarily the pin capacitance. R1 is typically 3250 Ω and is

a lumped component made up of some serial resistors and the

on resistance of the switches. C2 is typically 60 pF and is mainly

the ADC sampling capacitor. During the conversion phase,

where the switches are opened, the input impedance is limited

to C1. R1 and C2 make a 1-pole low-pass filter that reduces

undesirable aliasing effect and limits the noise.

When the source impedance of the driving circuit is low, the

AD7651 can be driven directly. Large source impedances will

significantly affect the ac performance, especially total

harmonic distortion.

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