AD7685ARMZ datasheet(16/29 Pages) AD

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Data Sheet

AD7685

ANALOG INPUTS

Figure 28 shows an equivalent circuit of the input structure of

the AD7685.

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 will cause these diodes to begin to

forward-bias 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, an input buffer with a short-circuit current limitation can

be used to protect the part.

CIN

RIN

CPIN

IN+

OR IN–

GND

VDD

Figure 28. Equivalent Analog Input Circuit

This analog input structure allows the sampling of the

differential signal between IN+ and IN−. By using this

differential input, small signals common to both inputs are

rejected, as shown in Figure 29, which represents the typical

CMRR over frequency. For instance, by using IN− to sense a

remote signal ground, ground potential differences between

the sensor and the local ADC ground are eliminated.

FREQUENCY (kHz)

10000

100

1000

VDD = 5V

VDD = 2.5V

Figure 29. 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 3 kΩ 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

AD7685 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, as shown

in Figure 30.

FREQUENCY (kHz)

100

–60

–70

–80

–90

–100

–110

–120

RS = 33Ω

RS = 50Ω

RS = 100Ω

RS = 250Ω

Figure 30. THD vs. Analog Input Frequency and Source Resistance

Rev. D | Page 15 of 28

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