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AD7621

Rev. 0 | Page 17 of 32

TYPICAL CONNECTION DIAGRAM

Figure 23 shows a typical connection diagram for the AD7621.

Different circuitry from that shown in this diagram are optional

and are discussed below.

ANALOG INPUTS

Figure 24 shows an equivalent circuit of the input structure of

the AD7621.

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

analog inputs, IN+ and IN−. Care must be taken to ensure that

the analog input signal never exceeds the supply rails by more

than 0.3 V as this causes the 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 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.

RIN

CIN

IN+ OR IN–

AGND

AVDD

CPIN

Figure 24. AD7621 Simplified Analog Input.

The analog input of AD7621 is a true differential structure. By

using this differential input, small signals common to both

inputs are rejected, as shown in Figure 25, representing the

typical CMRR over frequency with internal and external

references.

FREQUENCY (kHz)

100

10k

EXT REF

INT REF

Figure 25. Analog Input CMRR vs. Frequency

During the acquisition phase for ac signals, the impedance of

the analog inputs, IN+ and IN−, can be modeled as a parallel

combination of Capacitor CPIN and the network formed by the

series connection of RIN and CIN. CPIN is primarily the pin

capacitance. RIN is typically 350 Ω and is a lumped component

comprised of some serial resistors and the on resistance of the

switches. CIN is typically 12 pF and is mainly the ADC sampling

capacitor. During the conversion phase, when the switches are

opened, the input impedance is limited to CPIN. RIN and CIN

make a one-pole, low-pass filter that has a typical −3 dB cutoff

frequency of 50 MHz, thereby reducing an undesirable aliasing

effect while limiting noise from the inputs.

Since the input impedance of the AD7621 is very high, the

AD7621 can be directly driven by a low impedance source

without gain error. To further improve the noise filtering

achieved by the AD7621 analog input circuit, an external, one-

pole RC filter between the amplifier’s outputs and the ADC

analog inputs can be used, as shown in Figure 23. However,

large source impedances significantly affect the ac performance,

especially total harmonic distortion (THD). The maximum

source impedance depends on the amount of THD that can be

tolerated. The THD degrades as a function of the source

impedance and the maximum input frequency, as shown in

Figure 26.

INPUT FREQUENCY (kHz)

–105

–60

–65

–70

–75

–80

–85

–90

–95

–100

100

RS = 500Ω

RS = 50Ω

RS = 100Ω

RS = 10Ω

PDBUF = PDREF = LOW

Figure 26. THD vs. Analog Input Frequency and Source Resistance

DRIVER AMPLIFIER CHOICE

Although the AD7621 is easy to drive, the driver amplifier

needs to meet the following requirements:

•

Together, the driver amplifier and the AD7621 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 data sheet, settling at 0.1% to 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 op amp, which

combines ultralow noise and high gain bandwidth, meets

this settling time requirement even when used with gains

up to 13.

•

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 AD7621. The noise

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