AD7623ASTZRL View Datasheet(PDF) - Analog Devices

Part Name

Description

MFG CO.

AD7623ASTZRL

16-Bit, 1.33 MSPS PulSAR® ADC

Analog Devices 

AD7623

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

SNRLOSS

=

20

log

⎜⎛

⎜⎜⎝

53

⎟⎞

2809 + πf−3dB (NeN )2 ⎟⎟⎠

where:

f–3dB is the input bandwidth of the AD7623 (50 MHz) or the

cutoff frequency of the input filter (16 MHz), if one is used.

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

configuration).

eN 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 using it.

• The driver needs to have a THD performance suitable to

that of the AD7623. 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, produces 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 differen-

tial driver can be used.

ANALOG INPUT

(UNIPOLAR 0V TO 2.048V)

U1

AD8021

10pF

590Ω

590Ω

10Ω

1nF

1kΩ

1kΩ

100nF

U2

AD8021

10pF

10Ω

1nF

IN+

AD7623

IN– REF

10μF

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

(Internal Reference Buffer Used)

VOLTAGE REFERENCE INPUT

The AD7623 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 AD7623 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 AD7623 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.

Rev. 0 | Page 18 of 28

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