AD7663AST datasheet(15/25 Pages) AD

15 / 25 page

REV. B

AD7663

–14–

100

1000

FREQUENCY – kHz

Figure 7. Analog Input CMRR vs. Frequency

During the acquisition phase for ac signals, the AD7663 behaves

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

the resistive scaler R/2 in series with R1 and CS. The resistor R1

is typically 2700

W and is a lumped component 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

800 kHz reduces undesirable aliasing effects and limits the noise

coming from the inputs.

Except when using the 0 V to 2.5 V analog input voltage range,

the AD7663 has to be driven by a very low impedance source to

avoid gain errors. That can be done by using a driver amplifier

whose choice is eased by the primarily resistive analog input

circuitry of the AD7663.

When using the 0 V to 2.5 V analog input voltage range, the input

impedance of the AD7663 is very high so the AD7663 can be

driven directly by a low impedance source without gain error.

That allows, as shown in Figure 5, putting 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 by the

AD7663 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 degradation is a function of the source impedance

and the maximum input frequency as shown in Figure 8.

FREQUENCY – kHz

–70

100

1000

–80

–90

–100

–110

R = 100

R = 11

R = 50

Figure 8. THD vs. Analog Input Frequency and Input

Resistance (0 V to 2.5 V Only)

Driver Amplifier Choice

Although the AD7663 is easy to drive, the driver amplifier needs

to meet at least the following requirements:

• The driver amplifier and the AD7663 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% to 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

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 transition

noise performance of the AD7663. The noise coming from

the driver is first scaled down by the resistive scaler according

to the analog input voltage range used, and is then filtered by

the AD7663 analog input circuit one-pole, low-pass filter

made by (R/2 + R1) and CS. The SNR degradation due to

the amplifier is

SNR

FSR

LOSS

ËÁ

¯˜

784

log

–

where:

f–3 dB is the –3 dB input bandwidth in MHz of the AD7663

(0.8 MHz) or the cut-off frequency of the input filter

if any used (0 V to 2.5 V range).

is the noise factor of the amplifier (1 if in buffer

configuration).

is the equivalent input noise voltage of the op amp

in nV/Hz

1/2.

FSR is the full-scale span (i.e., 5 V for ±2.5 V range).

For instance, when using the 0 V to 2.5 V range, a driver

like the AD8610 with an equivalent input noise of 6 nV/

÷Hz

and configured as a buffer, thus with a noise gain of 1, the

SNR degrades by only 0.24 dB.

• The driver needs to have a THD performance suitable to

that of the AD7663. TPC 10 gives the THD versus frequency

that the driver should preferably exceed.

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 AD829 is another alternative where high frequency (above

100 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.

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