AD7679 datasheet(12/20 Pages) AD

12 / 20 page

REV. B

AD7676

–12–

During the acquisition phase for ac signals, the AD7676 behaves

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

R+, R–, and CS. The resistors R+ and R– are typically 684

Ω 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 unde-

sirable aliasing effects 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 (THD).

The maximum source impedance depends on the amount of

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 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 capaci-

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

the driver selection. The tiny op amp AD8021, which com-

bines ultralow noise and a high gain bandwidth, meets this

settling time requirement even when used with a high gain

up to 13.

• The driver needs to have a THD performance suitable to

that of the AD7676.

• The noise generated by the driver amplifier needs to be kept

as low as possible 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 degradation due to

the amplifier is:

SNR

LOG

fN e

LOSS













−

784

()

where:

f–3 dB is the –3 dB input bandwidth of the AD7676 (3.9 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

configuration).

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

√Hz.

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, will degrade the SNR by only 0.26 dB.

The AD8021 meets these requirements and is usually appropriate

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 a 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) performance is not required. In a 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.

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 imped-

ance source with an efficient decoupling between the 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 appropriate 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.

Care should also be taken with the reference temperature coeffi-

cient of the voltage reference, which directly affects the full-scale

accuracy if this parameter matters. For instance, a

±15 ppm/°C

tempco of the reference changes the full scale by

±1 LSB/°C.

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