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Data Sheet

AD7982

Rev. D | Page 15 of 25

ANALOG INPUTS

Figure 24 shows an equivalent circuit of the input structure of the

AD7982.

The two diodes, D1 and D2, provide electrostatic discharge

(ESD) protection for the IN+ analog input and the IN− analog

input. Take care to ensure the analog input signal does not

exceed the reference input voltage (REF) by more than 0.3 V. If

the analog input signal exceeds the 0.3 V level, the diodes

become forward-biased and begin conducting current. These

diodes can handle a forward-biased current of 130 mA maximum.

However, if the supplies of the input buffer (for example, the

supplies of the ADA4807-1 in Figure 23) are different from those of

the REF, the analog input signal can eventually exceed the

supply rails by more than 0.3 V. In such a case (for example, an

input buffer with a short-circuit), the current limitation can protect

the device.

CPIN

REF

RIN

CIN

IN+ OR IN–

GND

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.

100

1000

10000

FREQUENCY (kHz)

Figure 25. Analog Input CMRR vs. Frequency

During the acquisition phase, the impedance of the analog

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

of Capacitor CPIN and the network formed by the series connection

of RIN and CIN. CPIN is primarily the pin capacitance. RIN is typically

400 Ω and is a lumped component composed of serial resistors

and the on resistance of the switches. CIN is typically 30 pF and

is mainly the ADC sampling capacitor.

During the sampling phase where the switches are closed, the input

impedance is limited to CPIN. RIN and CIN make a 1-pole, low-pass

filter that reduces undesirable aliasing effects and limits noise.

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

AD7982 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 AD7982 is easy to drive, the driver amplifier must

meet the following requirements:



The noise generated by the driver amplifier must be kept

as low as possible to preserve the SNR and transition noise

performance of the AD7982. The noise from the driver is

filtered by the analog input circuit of the AD7982 1-pole,

low-pass filter made by RIN and CIN, or by the external

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

AD7982 is 40 μV rms, the SNR degradation due to the

amplifier is



















)

(

log

3dB

LOSS

SNR

where:

f–3dB is the input bandwidth, in megahertz, of the AD7982

(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).

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

nV/√Hz.



For ac applications, the driver must have a THD perfor-

mance commensurate with the AD7982.



For multichannel, multiplexed applications, the driver

amplifier and the AD7982 analog input circuit must settle

for a full-scale step onto the capacitor array at an 18-bit level

(0.0004%, 4 ppm). In the data sheet of the amplifier, settling at

0.1% to 0.01% is more typically specified. Settling time can

differ significantly from the settling time at an 18-bit level

and must be verified prior to driver selection.

Table 9. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4941-1

Very low noise, low power, single to differential

ADA4940-1

Very low noise, low power, single to differential

ADA4807-2

Very low noise and low power

ADA4627-1

Precision, low noise and low input bias

ADA4522-2

Precision, zero drift, and electromagnetic

interference (EMI) enhanced

ADA4500-2

Precision, rail-to-rail input and output (RRIO), and

zero input crossover distortion

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