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AD7654

Data Sheet

Rev. D | Page 16 of 27

TYPICAL CONNECTION DIAGRAM

Figure 19 shows a typical connection diagram for the AD7654.

Different circuitry shown on this diagram is optional and is

discussed in the following sections.

ANALOG INPUTS

Figure 20 shows a simplified analog input section of the AD7654.

INA1

INB2

AGND

AVDD

INA2

INAN

INBN

INB1

A0 = L

A0 = H

Figure 20. Simplified Analog Input

The diodes shown in Figure 20 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

these diodes to become forward biased and start conducting

current. These diodes can handle a forward-biased current of

120 mA maximum. This condition can eventually occur when

the input buffers (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 device.

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 these

differential inputs, small signals common to both inputs are

rejected.

During the acquisition phase, for ac signals, the AD7654 behaves

like a one-pole RC filter consisting of the equivalent resistance RA,

RB, and CS. 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 capacitor CS 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 AD7654 is very high, the

AD7654 can be driven directly by a low impedance source

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

AD7654 analog input circuit, an external one-pole RC filter

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

Figure 19, 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 as the source impedance

increases.

INPUT CHANNEL MULTIPLEXER

The AD7654 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 23 for timing details.

DRIVER AMPLIFIER CHOICE

Although the AD7654 is easy to drive, the driver amplifier

needs to meet at least the following requirements:



For multichannel, multiplexed applications, the driver

amplifier and the AD7654 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 amplifier’s data

sheet, the settling at 0.1% or 0.01% is more commonly

specified. It can significantly differ from the settling time at

a 16-bit level and, therefore, it should be verified prior to

the driver selection.



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 AD7654. The noise coming from

the driver is filtered by the AD7654 analog input circuit

one-pole low-pass filter made by RA, RB, and CS. The SNR

degradation due to the amplifier is





















)

(

log

LOSS

SNR

where:

f–3 dB is the –3 dB input bandwidth in MHz of the AD7654

(10 MHz) or the cutoff frequency of the input filter, if any is

used.

N is the noise factor of the amplifier (1 if in buffer

configuration).

eN is the equivalent input noise voltage of the op amp in

nV/√Hz.

For instance, a driver like the AD8021 with an equivalent

input noise of 2 nV/√Hz, configured as a buffer, and thus

with a noise gain of +1, degrades the SNR by only 0.06 dB

with the filter in Figure 19, and by 0.10 dB without.



The driver needs to have a THD performance suitable to

that of the AD7654.

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