AD7983

Data Sheet

Rev. B | Page 14 of 24

ANALOG INPUTS

Figure 24 shows an equivalent circuit of the input structure of

the AD7983.

The two diodes, D1 and D2, provide ESD protection for the

analog inputs, IN+ and IN−. Care must be taken to ensure that

the analog input signal never exceeds the supply rails by more

than 0.3 V, because this causes these diodes to become forward-

biased and start conducting current. These diodes can handle a

forward-biased current of 130 mA maximum. For instance,

these conditions could eventually occur when the supplies of

the input buffer (U1) are different from VDD. In such a case

(for example, an input buffer with a short circuit), the current

limitation can be used to protect the part.

REF

IN+

OR IN–

GND

D2

D1

Figure 24. Equivalent Analog Input Circuit

The analog input structure allows the sampling of the true

differential signal between IN+ and IN−. By using these

differential inputs, signals common to both inputs are rejected.

During the acquisition phase, the impedance of the analog

inputs (IN+ and IN−) can be modeled as a parallel combination of

400 Ω and is a lumped component made up of some serial

30 pF and is mainly the ADC sampling capacitor. During the

conversion phase, where the switches are opened, the input

filter that reduces undesirable aliasing effects and limits the noise.

When the source impedance of the driving circuit is low, the

AD7983 can be driven directly. Large source impedances

significantly affect the ac performance, especially THD. The dc

performances are less sensitive to the input impedance. The

maximum source impedance depends on the amount of THD

that can be tolerated. The THD degrades as a function of the

source impedance and the maximum input frequency.

DRIVER AMPLIFIER CHOICE

Although the AD7983 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 AD7983. The noise coming from

the driver is filtered by the AD7983 analog input circuit’s

filter, if one is used. Because the typical noise of the AD7983

is 39.7 µV rms, the SNR degradation due to the amplifier is

























+

=

−

2

3dB

2

)

(

2

π

7

.

9

3

39.7

log

20

N

LOSS

Ne

f

SNR

where:

(10 MHz) or the cutoff frequency of the input filter, if

one is used.

N is the noise gain of the amplifier (for example, 1 in buffer

configuration).

in nV/√Hz.

•

For ac applications, the driver should have a THD

performance commensurate with the AD7983.

•

For multichannel multiplexed applications, the driver

amplifier and the AD7983 analog input circuit must settle

for a full-scale step onto the capacitor array at a 16-bit level

(0.0015%, 15 ppm). In the data sheet of the amplifier, settling

at 0.1% to 0.01% is more commonly specified. This could

differ significantly from the settling time at a 16-bit level

and should be verified prior to driver selection.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4841-x

Very low noise, small and low power

AD8021

Very low noise and high frequency

AD8022

Low noise and high frequency

OP184

Low power, low noise, and low frequency

AD8655

5 V single-supply, low noise

AD8605, AD8615

5 V single-supply, low power