REV. A

AD7675

–12–

During the acquisition phase for ac signals, the AD7675 behaves

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

Ω

and are lumped components made up of some serial resistors

60 pF and is mainly the ADC sampling capacitor. This one pole

filter with a typical –3 dB cutoff frequency of 3.9 MHz reduces

undesirable aliasing effect and limits the noise coming from

the inputs.

Because the input impedance of the AD7675 is very high, the

AD7675 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 AD7675 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.

U2

590

590

2.5V REF

AD8021

590

AD7675

IN+

IN–

REF

2.5V REF

U1

ANALOG INPUT

(UNIPOLAR)

AD8021

590

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

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 AD7675 is easy to drive, the driver amplifier needs

to meet at least the following requirements:

• The driver amplifier and the AD7675 analog input circuit

have to be able 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 speci-

fied. 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 transi-

tion noise performance of the AD7675. The noise coming

from the driver is filtered by the AD7675 analog input circuit

degradation due to the amplifier is:

SNR

LOG

fN e

LOSS

dB

N

=

+





















20

28

784

4

3

2

π

–

()

where

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)

nV/(Hz)

For instance, in the case of 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 0.04 dB

with the filter in Figure 5, and 0.07 dB without.

• The driver needs to have a THD performance suitable to

that of the AD7675.

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 differ-

ential signal from a single-ended signal. When using the AD8138

with the filter in Figure 5, the SNR degrades by only 0.9 dB.

The AD829 is another alternative where high frequency (above

100 kHz) performances are 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.

The AD8519, OP162, or the OP184 could also be used.

Voltage Reference Input

The AD7675 uses an external 2.5 V voltage reference.

The voltage reference input REF of the AD7675 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 1

µF ceramic

capacitor and a low ESR tantalum capacitor connected to the

REF and REFGND inputs with minimum parasitic induc-

tance. 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 AD7675s, it is more effective to

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

like the AD8031.