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AD7690

Data Sheet

ANALOG INPUTS

Figure 27 shows an equivalent circuit of the input structure of

the AD7690.

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 supply rails by more

than 0.3 V because this causes the 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 input buffer’s (U1)

supplies 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.

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

of the capacitor, CPIN, and the network formed by the series

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

RIN is typically 600 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 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

AD7690 can be driven directly. Large source impedances

significantly affect the ac performance, especially total

harmonic distortion (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.

Figure 29. THD vs. Analog Input Frequency and Source Resistance

DRIVER AMPLIFIER CHOICE

Although the AD7690 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 AD7690. The noise from the driver is

filtered by the AD7690 analog input circuit’s 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 AD7690 is

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

















−

)

(

)

(

log

LOSS

SNR

where:

f−3dB is the input bandwidth in megahertz of the AD7690

(9 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+ and eN− are the equivalent input noise voltage densities

of the op amps connected to IN+ and IN−, in nV/√Hz.

This approximation can be used when the resistances

around the amplifiers are small. If larger resistances are

used, their noise contributions should also be root

summed squared.

•

For ac applications, the driver should have a THD

performance commensurate with the AD7690.

CIN

RIN

CPIN

IN+

OR IN–

GND

VDD

10000

FREQUENCY (kHz)

100

1000

VREF = VDD = 5V

FREQUENCY (kHz)

–130

–125

–120

–115

–110

–105

–100

–95

–90

–85

–80

33Ω

100Ω

15Ω

VREF = VDD 5V

250Ω

50Ω

Rev. C | Page 14 of 24

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