AD79821 datasheet(14/24 Pages) AD

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AD7946

Rev. A | Page 14 of 24

tolerated. The THD degrades as a function of the source

impedance and the maximum input frequency, as shown in

Figure 28.

FREQUENCY (kHz)

–105

–110

–100

–90

–95

–85

–80

255075

100

RS = 33Ω

RS = 50Ω

RS =100Ω

RS = 250Ω

Figure 28. THD vs. Analog Input Frequency and Source Resistance

DRIVER AMPLIFIER CHOICE

Although the AD7946 is easy to drive, the driver amplifier

needs to meet the following requirements:

•

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 AD7946. Note that the AD7946

has a noise much lower than most of the other 14-bit

ADCs and, therefore, can be driven by a noisier amplifier

to meet a given system noise specification. The noise

coming from the amplifier is filtered by the AD7946 analog

input circuit 1-pole, low-pass filter made by RIN and CIN or

by the external filter, if one is used.

•

For ac applications, the driver should have a THD perform-

ance commensurate with the AD7946. Figure 17 shows the

THD vs. frequency that the driver should exceed.

•

For multichannel multiplexed applications, the driver

amplifier and the AD7946 analog input circuit must settle a

full-scale step onto the capacitor array at a 14-bit level

(0.006%). In the amplifier’s data sheet, settling at 0.1% to

0.01% is more commonly specified. This could differ

significantly from the settling time at a 14-bit level and

should be verified prior to driver selection.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4841

Very low noise, small and low power

AD8021

Very low noise and high frequency

AD8655

5 V single-supply, low noise, and low power

AD8022

Low noise and high frequency

OP184

Low power, low noise, and low frequency

AD8605, AD8615

5 V single-supply, low power

AD8519

Small, low power, and low frequency

AD8031

High frequency and low power

VOLTAGE REFERENCE INPUT

The AD7946 voltage reference input REF has a dynamic input

impedance and should therefore be driven by a low impedance

source with efficient decoupling between the REF and GND

pins, as explained in the Layout section.

When REF is driven by a very low impedance source, for

example, a reference buffer using the AD8031 or the AD8603, a

10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate for

optimum performance.

If an unbuffered reference voltage is used, the decoupling value

depends on the reference used. For instance, a 22 μF (X5R,

1206 size) ceramic chip capacitor is appropriate for optimum

performance using a low temperature drift ADR43x reference.

If desired, smaller reference decoupling capacitor values down

to 2.2 μF can be used with a minimal impact on performance,

especially DNL.

Regardless, there is no need for an additional lower value ceramic

decoupling capacitor (for example, 100 nF) between the REF

and GND pins.

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