AD7655AST datasheet(16/28 Pages) AD

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AD7655

Rev. B | Page 16 of 28

TYPICAL CONNECTION DIAGRAM

Figure 17 shows a typical connection diagram for the AD7655.

Some of the circuitry shown in this diagram is optional and is

discussed in the following sections.

ANALOG INPUTS

Figure 18 shows a simplified analog input section of the

AD7655.

INA1

INB2

AGND

AVDD

INA2

INAN

INBN

INB1

A0 = L

A0 = H

Figure 18. Simplified Analog Input

The diodes shown in Figure 18 provide ESD protection for

the inputs. Care must be taken to ensure that the analog input

signal never exceeds the absolute ratings on these inputs.

This causes the diodes to become forward biased and start

conducting current. These diodes can handle a forward-biased

current of 120 mA maximum. This condition can occur when

the input buffer (U1) or (U2) 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.

This analog input structure allows the sampling of the

differential signal between INx and INxN. Unlike other

converters, the INxN is sampled at the same time as the INx

input. By using differential inputs, small signals common to

both inputs are rejected.

During the acquisition phase, for ac signals, the AD7655

behaves like a one-pole RC filter consisting of the equivalent

resistance RA, RB, and C

. The resistors RA and RB

are typically

500 Ω and are a lumped component made up of some serial

resistors and the on resistance of the switches. The CS capacitor

is typically 32 pF and is mainly the ADC sampling capacitor.

This one-pole filter with a typical −3 dB cutoff frequency of

10 MHz reduces undesirable aliasing effects and limits the

noise coming from the inputs.

Because the input impedance of the AD7655 is very high, the

AD7655 can be driven directly by a low impedance source

without gain error. To further improve the noise filtering of the

AD7655 analog input circuit, an external, one-pole RC filter

between the amplifier output and the ADC input, as shown in

Figure 17, can be used. However, the source impedance has to

be kept low because it affects the ac performance, especially the

total harmonic distortion. The maximum source impedance

depends on the amount of total harmonic distortion (THD)

that can be tolerated. The THD degrades when the source

impedance increases.

INPUT CHANNEL MULTIPLEXER

The AD7655 allows the choice of simultaneously sampling the

inputs pairs INA1/INB1 or INA2/INB2 with the A0 multiplexer

input. When A0 is low, the input pairs INA1/INB1 are selected,

and when A0 is high, the input pairs INA2/INB2 are selected.

Note that INAx is always converted before INBx regardless of

the state of the digital interface channel selection A/B pin.

Also note that the channel selection control, A0, should not be

changed during the acquisition phase of the converter. Refer to

the Conversion Control section and Figure 21 for timing details.

DRIVER AMPLIFIER CHOICE

Although the AD7655 is easy to drive, the driver amplifier

needs to meet at least the following requirements:

•

The noise generated by the driver amplifier needs to be

kept as low as possible to preserve the SNR and transition

noise performance of the AD7655. The noise coming from

the driver is filtered by the AD7655 analog input circuit

one-pole, low-pass filter made by RA, RB, and C

or by an

external filter, if one is used.

•

The driver needs to have a THD performance suitable to

that of the AD7655.

•

For multichannel, multiplexed applications, the driver

amplifier and the AD7655 analog input circuit together

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

array at a 16-bit level (0.0015%). In the data sheet for the

driver amplifier, the settling at 0.1% or 0.01% is more

commonly specified. This could differ significantly from

the settling time at a 16-bit level and should be verified

prior to driver selection.

The AD8021 meets these requirements and, for almost all

applications, is usually appropriate. The AD8021 needs an

external compensation capacitor of 10 pF. This capacitor should

have good linearity as an NPO ceramic or mica type. The

AD8022 can be used where a dual version is needed and a gain

of +1 is used.

The AD829 is another alternative where high frequency (above

100 kHz) performance is not required. In a gain of +1, it

requires an 82 pF NPO or mica type compensation capacitor.

The AD8610 is another option where low bias current is needed

in low frequency applications.

Refer to Table 8 for some recommended op amps.

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