AD7676AST datasheet(12/20 Pages) AD

12 / 20 page

REV. 0

AD7676

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During the acquisition phase, for ac signals, the AD7676 behaves

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

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

684 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

3.88 MHz reduces undesirable aliasing effect and limits the

noise coming from the inputs.

Because the input impedance of the AD7676 is very high, the

AD7676 can be driven directly by a low impedance source

without gain error. That allows users to put, 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 AD7676 analog

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

low because it affects the ac performances, 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 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

AD7676

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 a common mode at 1.25 V.

If the application can tolerate more noise, the AD8138 can be used.

Driver Amplifier Choice

Although the AD7676 is easy to drive, the driver amplifier needs

to meet at least the following requirements:

• The driver amplifier and the AD7676 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

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

driver selection. The tiny op amp AD8021 which combines

ultra low 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 transition

noise performance of the AD7676. The noise coming from the

driver is filtered by the AD7676 analog input circuit one-pole,

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

tion due to the amplifier is :

where

f–3 dB is the –3 dB input bandwidth of the AD7676 (3.88 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 con-

figuration)

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

nV/(Hz)

1/2.

FSR is 2

× VREF.

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.

• The driver needs to have a THD performance suitable to

that of the AD7676.

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 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

500 kHz) performances is not required. In gain of 1, it requires

a 82 pF compensation capacitor.

The AD8610 is also another option where low-bias current is

needed in low-frequency applications.

Voltage Reference Input

The AD7676 uses an external 2.5 V voltage reference.

The voltage reference input REF of the AD7676 has a dynamic

input impedance. Therefore, it should be driven by a low-

impedance source with an efficient decoupling between REF

and REFGND inputs. This decoupling depends on the choice

of the voltage reference but usually consists of a low ESR

tantalum capacitor connected to the REF and REFGND

inputs with minimum parasitic inductance. 47

µF is an appro-

priate value for the tantalum capacitor when used with one of

the recommended reference voltages:

• The low-noise, low temperature drift ADR421 and AD780

voltage references

• The low-power ADR291 voltage reference

• The low-cost AD1582 voltage reference

For applications using multiple AD7676s, it is more effective to

buffer the reference voltage with a low-noise, very stable op amp

like the AD8031.

SNR

LOG

fN e

LOSS













784

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