AD7677 datasheet(13/20 Pages) AD

13 / 20 page

REV. 0

AD7677

–13–

FREQUENCY – Hz

10k

10M

100k

Figure 7. Analog Input CMRR vs. Frequency

During the acquisition phase, for ac signals, the AD7677 behaves

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

tance R+ , R–, and CS. The resistors R+ and R– are typically

168 V and are lumped components made up of some serial

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

typically 60 pF and is mainly the ADC sampling capacitor. This

one-pole filter with a typical –3 dB cutoff frequency of 15.8 MHz

reduces undesirable aliasing effect and limits the noise com-

ing from the inputs.

Because the input impedance of the AD7677 is very high, the

AD7677 can be driven directly by a low impedance source

without gain error. That allows the user to input, as shown in

Figure 5, an external one-pole RC filter between the output of

the amplifier output and the ADC analog inputs to even further

improve the noise filtering done by the AD7677 analog input

circuit. However, the source impedance has to be kept low

because it affects the ac performances, especially the total har-

monic distortion. The maximum source impedance depends

on the amount of total harmonic distortion (THD) that can

be tolerated. The THD degrades proportionally to the source

impedance.

Single to Differential Driver

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

to-differential driver will allow for a differential input into the

part. The schematic is shown in Figure 8.

590

2.5V REF

AD8021

590

AD7677

IN+

IN–

REF

2.5V REF

ANALOG INPUT

(UNIPOLAR)

AD8021

590

Figure 8. Single-Ended-to-Differential Driver Circuit

This configuration, when provided an input signal of 0 to VREF,

will produce a differential

±2.5 V with midscale at 1.25 V.

If the application can tolerate more noise, the AD8138 can

be used.

Driver Amplifier Choice

Although the AD7677 is easy to drive, the driver amplifier needs

to meet at least the following requirements:

• The driver amplifier and the AD7677 analog input circuit

have to be able together 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 could significantly differ from the settling time at

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

driver selection. The tiny op-amp, AD8021, which combines

ultralow noise and a high gain bandwidth, meets this settling

time requirement even when used with a high gain 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 transi-

tion noise performance of the AD7677. The noise coming

from the driver is filtered by the AD7677 analog input circuit

one-pole, low-pass filter made by R+, R–, and CS. The SNR

degradation due to the amplifier is:

SNR

LOG

fN e

LOSS

()

















784

–

where

f–3 dB is the –3 dB input bandwidth in MHz of the AD7677

(15.8 MHz) or the cutoff frequency of the input filter if

any used.

N is the noise factor of the amplifiers (1 if in buffer con-

figuration).

eN is the equivalent input noise voltage of each opamp in

nV/(Hz)

1/2.

For instance, a driver with an equivalent input noise of 2 nV/

√Hz

(like the AD8021) and configured as a buffer, thus with a noise

gain of +1, the SNR degrades by only 0.07 dB with the filter in

Figure 5, and 0.27 dB without.

• The driver needs to have a THD performance suitable to

that of the AD7677.

The AD8021 meets these requirements and is usually appropri-

ate for almost all applications. 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 could also be used where a dual version is needed

and gain of 1 is used.

The AD8132 or the AD8138 could also be used to generate a

differential signal from a single-ended signal.

The AD829 is another alternative where high-frequency (above

1 MHz) performance is not required. In gain of 1, it requires an

82 pF compensation capacitor.

The AD8610 is also another option where low bias current is

needed in low-frequency applications.

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