AD7665 datasheet(14/23 Pages) AD

14 / 23 page

REV.

AD7665

–14–

Analog Inputs

The AD7665 is specified to operate with six full-scale analog input

ranges. Connections required for each of the four analog inputs,

IND, INC, INB, and INA, and the resulting full-scale ranges are

shown in Table I. The typical input impedance for each analog

input range is also shown.

Figure 6 shows a simplified analog input section of the AD7665.

The four resistors connected to the four analog inputs form a

resistive scaler that scales down and shifts the analog input range

to a common input range of 0 V to 2.5 V at the input of the

switched capacitive ADC.

INC

INB

INA

IND

AGND

AVDD

R = 1.28k

Figure 6. Simplified Analog Input

By connecting the four inputs INA, INB, INC, and IND to the

input signal itself, the ground, or a 2.5 V reference, other analog

input ranges can be obtained.

The diodes shown in Figure 6 provide ESD protection for the

four analog inputs. The inputs INB, INC, and IND have a high

voltage protection (–11 V to +30 V) to allow a wide input voltage

range. Care must be taken to ensure that the analog input signal

never exceeds the absolute ratings on these inputs, including

INA (0 V to 5 V). This will cause these diodes to become forward-

biased and start conducting current. These diodes can handle a

forward-biased current of 120 mA maximum. For instance, when

using the 0 V to 2.5 V input range, these conditions could eventu-

ally occur on the input INA when the input buffer’s (U1) supplies

are different from AVDD. In such cases, an input buffer with a

short circuit current limitation can be used to protect the part.

100

1000

10000

FREQUENCY – kHz

Figure 7. Analog Input CMRR vs. Frequency

This analog input structure allows the sampling of the differential

signal between the output of the resistive scaler and INGND.

Unlike other converters, the INGND input is sampled at the same

time as the inputs. By using this differential input, small signals

common to both inputs are rejected as shown in Figure 7, which

represents the typical CMRR over frequency. For instance, by using

INGND to sense a remote signal ground, the difference of ground

potentials between the sensor and the local ADC ground is eliminated.

During the acquisition phase for ac signals, the AD7665 behaves

like a one-pole RC filter consisting of the equivalent resistance

of the resistive scaler R/2 in series with R1 and CS. The resistor

R1 is typically 100

W and is a lumped component made up of

some serial resistors and the on resistance of the switches. The

capacitor CS is typically 60 pF and is mainly the ADC sampling

capacitor. This one-pole filter with a typical –3 dB cutoff frequency

of 3.6 MHz reduces undesirable aliasing effects and limits the

noise coming from the inputs.

Except when using the 0 V to 2.5 V analog input voltage range, the

AD7665 has to be driven by a very low impedance source to avoid

gain errors. That can be done by using a driver amplifier whose

choice is eased by the primarily resistive analog input circuitry of

the AD7665.

When using the 0 V to 2.5 V analog input voltage range, the input

impedance of the AD7665 is very high so the AD7665 can be

driven directly by a low impedance source without gain error.

That allows, as shown in Figure 5, putting an external one-pole

RC filter between the output of the amplifier output and the ADC

analog inputs to even further improve the noise filtering done by

the AD7665 analog input circuit. However, the source impedance

has to be kept low because it affects the ac performances, especially

the total harmonic distortion (THD). The maximum source

impedance depends on the amount of total THD that can be

tolerated. The THD degradation is a function of the source imped-

ance and the maximum input frequency as shown in Figure 8.

FREQUENCY – kHz

–110

100

–100

–90

–80

–70

1000

R = 50

R = 11

R = 100

Figure 8. THD vs. Analog Input Frequency and Input

Resistance (0 V to 2.5 V Only)

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