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AD7621

Rev. 0 | Page 18 of 32

coming from the driver is filtered by the AD7621 analog

input circuit one-pole, low-pass filter made by RIN and CIN

or by the external filter, if one is used. The SNR

degradation due to the amplifier is

() ⎟⎟

⎟

⎠

⎞

⎜⎜

⎜

⎝

⎛

−

2809

log

LOSS

SNR

where:

f–3dB is the input bandwidth of the AD7621 (50 MHz) or

the cutoff frequency of the input filter (16 MHz), if

one is used.

is the noise factor of the amplifier (+1 in buffer

configuration).

is the equivalent input voltage noise density of the op

amp, in nV/√Hz.

For instance, a driver with an equivalent input noise

density of 2.1 nV/√Hz, like the AD8021 with a noise gain

of +1 when configured as a buffer, degrades the SNR by

only 0.33 dB when using the RC filter in Figure 23, and by

1 dB without.

•

The driver needs to have a THD performance suitable to

that of the AD7621. Figure 13 gives the THD vs. frequency

that the driver should exceed.

The AD8021 meets these requirements and is appropriate for

almost all applications. The AD8021 needs a 10 pF external

compensation capacitor that 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.

The AD8022 can also be used when a dual version is needed

and a gain of 1 is present. The AD829 is an alternative in

applications where high frequency (above 100 kHz) performance

is not required. In applications with a gain of 1, an 82 pF

compensation capacitor is required. The AD8610 is an option

when low bias current is needed in low frequency applications.

Single-to-Differential Driver

For applications using unipolar analog signals, a single-ended-

to-differential driver, as shown in Figure 27, allows for a

differential input into the part. This configuration, when

provided an input signal of 0 to VREF, will produce a differential

±VREF with midscale at VREF/2. The one-pole filter using R = 10 Ω

and C = 1 nF provides a corner frequency of 16 MHz.

If the application can tolerate more noise, the AD8139

differential driver can be used.

AD8021

ANALOG INPUT

(UNIPOLAR 0V TO 2.048V)

AD8021

IN+

IN–

AD7621

REF

μF

100nF

1nF

10pF

590

Figure 27. Single-Ended-to-Differential Driver Circuit

(Internal Reference Buffer Used)

VOLTAGE REFERENCE INPUT

The AD7621 allows the choice of either a very low temperature

drift internal voltage reference or an external reference.

Unlike many ADCs with internal references, the internal

reference of the AD7621 provides excellent performance and

can be used in almost all applications.

Internal Reference

(PDBUF = Low, PDREF = Low)

To use the internal reference, the PDREF and PDBUF inputs

must be low. This produces a 1.2 V band gap output on

REFBUFIN which, amplified by the internal buffer, results in a

2.048 V reference on the REF pin.

The internal reference is temperature-compensated to 2.048 V ±

10 mV. The reference is trimmed to provide a typical drift of

7 ppm/°C. This typical drift characteristic is shown in Figure 7.

The output resistance of the REFBUFIN is 6.33 kΩ (minimum)

when the internal reference is enabled. It is necessary to

decouple this with a ceramic capacitor greater than 100 nF.

Thus, the capacitor provides an RC filter for noise reduction.

Since the output impedance of REFBUFIN is typically 6.33 kΩ,

relative humidity (among other industrial contaminates) can

directly affect the drift characteristics of the reference. Typically,

a guard ring is used to reduce the effects of drift under such

circumstances. However, since the AD7621 has a fine lead pitch,

guarding this node is not practical. Therefore, in these

industrial and other types of applications, it is recommended to

use a conformal coating such as Dow Corning 1-2577 or

Humiseal 1B73.

External 1.2 V Reference and Internal Buffer

(PDREF = High, PBBUF = Low)

To use an external reference with the internal buffer, PDREF

should be high and PDBUF should be low. This powers down

the internal reference and allows the 1.2 V reference to be

applied to REFBUFIN.

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