AD7980BCPZ-R2 datasheet(18/28 Pages) AD

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AD7980

Data Sheet

Rev. F | Page 16 of 26

ANALOG INPUT

Figure 28 shows an equivalent circuit of the input structure of

the AD7980.

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 device.

Figure 28. 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

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 made up of some serial

resistors and the on resistance of the switches. CIN is typically

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

conversion phase, where the switches are opened, the input

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

filter that reduces undesirable aliasing effects and limits the noise.

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

AD7980 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 AD7980 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 AD7980. The noise coming from

the driver is filtered by the 1-pole, low-pass filter of the

AD7980 analog input circuit made by RIN and CIN or by the

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

AD7980 is 47.3 μV rms, the SNR degradation due to the

amplifier is



















3dB

)

(

47.3

log

LOSS

SNR

where:

f–3dB is the input bandwidth in MHz of the AD7980

(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 should have a THD

performance commensurate with the AD7980.



For multichannel multiplexed applications, the driver

amplifier and the AD7980 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 amplifier data sheet, settling at

0.1% to 0.01% is more commonly specified. This can differ

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

should be verified prior to driver selection.

Table 9. Recommended Driver Amplifiers1

Amplifier

Typical Application

ADA4805-1

Low noise, small size, and low power

ADA4807-1

Very low noise and high frequency

ADA4627-1

Precision, low noise, and low input bias current

ADA4522-1

Precision, zero drift, and EMI enhanced

ADA4500-2

Precision, rail-to-rail input/output, and zero input

crossover distortion

1 For the latest recommended drivers, see the product recommendations

listed on the product webpage.

REF

RIN

CIN

IN+

OR IN–

GND

CPIN

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