Data Sheet

AD7989-1/AD7989-5

Rev. B | 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 electrostatic discharge (ESD)

protection for the IN+ analog input and IN− analog input.

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 this 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 26)

are different from those of 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.

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

400 Ω and is a lumped component composed of serial resistors

is mainly the ADC sampling capacitor.

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

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

typical noise of the AD7989-1/AD7989-5 is 40 μV rms, the

SNR degradation due to the amplifier is































2

2

)

(

2

π

40

40

log

20

N

3dB

LOSS

Ne

f

SNR



where:

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).

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 can differ significantly from the settling

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

selection.

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

listed on the product webpage.

REF

D1

D2

IN+ OR IN–

GND

90

85

80

75

70

65

60

1

10

100

1k

10k

FREQUENCY (kHz)