AD7989-5BCPZ-R2 datasheet(15/24 Pages) ETC2

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Data Sheet

AD7989-1/AD7989-5

Rev. 0 | Page 15 of 24

ANALOG INPUTS

Figure 27 shows an equivalent circuit of the input structure of

the AD7989-1/AD7989-5.

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 does not exceed the reference input

voltage (REF) by more than 0.3 V. If the analog input signal

exceeds this level, the diodes become forward-biased and start

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 ADA4841-x in

Figure 26) are different from those of REF, the analog input

signal may 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 be used to protect the device.

Figure 27. 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.

Figure 28. 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, when the switches are closed, the input

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

pass filter that reduces undesirable aliasing effects and limits

noise.

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

AD7989-1/AD7989-5 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 AD7989-1/AD7989-5 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 AD7989-1/AD7989-5. The noise from

the driver is filtered by the one-pole, low-pass filter of the

AD7989-1/AD7989-5 analog input circuit made by RIN and

CIN or by the external filter, if one is used. Because the

typical noise of the AD7989-1/AD7989-5 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 AD7989-1/

AD7989-5 (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, use a driver with a THD performance

commensurate with the AD7989-1/AD7989-5.

•

For multichannel multiplexed applications, the driver

amplifier and the AD7989-1/AD7989-5 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 commonly

specified. This settling may differ significantly from the

settling time at an 18-bit level and must be verified prior to

driver selection.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4941-1

Very low noise, low power, single to differential

ADA4841-1/

ADA4841-2

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

CPIN

REF

RIN

CIN

IN+ OR IN–

GND

100

10k

FREQUENCY (kHz)

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