AD7940 datasheet(14/24 Pages) AD

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AD7693

Preliminary Technical Data

Rev. PrB | Page 14 of 24

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

AD7693 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.

ANALOG INPUTS

Figure 27 shows an equivalent circuit of the input structure of

the AD7693.

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.

DRIVER AMPLIFIER CHOICE

Although the AD7693 is easy to drive, the driver amplifier must

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 AD7693. The noise coming from

the driver is filtered by the AD7693 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 AD7693 is 56 μV rms, the SNR degradation due to the

amplifier is

CIN

RIN

CPIN

IN+

OR IN–

GND

VDD

⎟⎟

⎟

⎠

⎞

⎜⎜

⎜

⎝

⎛

−

)

(

)

(

log

LOSS

SNR

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.

where:

f−3 dB is the input bandwidth in megahertz of the AD7693

(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 is the equivalent input noise voltage of the op amp, in

nV/√Hz.

10000

FREQUENCY (kHz)

100

1000

VREF = VDD = 5V

•

For ac applications, the driver should have a THD

performance commensurate with the AD7693.

•

For multichannel multiplexed applications, the driver

amplifier and the AD7693 analog input circuit must settle

for a full-scale step onto the capacitor array at an 16-bit

level (0.0015%, 15 ppm). In the amplifier’s data sheet,

settling at 0.1% to 0.01% is more commonly specified. This

could differ significantly from the settling time at an 16-bit

level and should be verified prior to driver selection.

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 made up of

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

typically 30 pF and is mainly the ADC sampling capacitor.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4941-1

Very low noise, low power single to differential

ADA4841-x

Very low noise, small, and low power

AD8655

5 V single supply, low noise

AD8021

Very low noise and high frequency

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.

AD8022

Low noise and high frequency

OP184

Low power, low noise, and low frequency

AD8605, AD8615

5 V single supply, low power

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