AD7679 datasheet(14/24 Pages) AD

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AD7671

–14–

Analog Inputs

The AD7671 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 AD7671.

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 =

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

eventually 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.

This analog input structure allows the sampling of the differen-

tial 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

AD7671 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 resis-

tance 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 9.6 MHz reduces undesirable aliasing effects

and limits the noise coming from the inputs.

100

1000

10000

FREQUENCY – kHz

Figure 7. Analog Input CMRR vs. Frequency

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

the AD7671 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 AD7671.

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

impedance of the AD7671 is very high so the AD7671 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 AD7671 analog input circuit. However, the source

impedance has to be kept low because it affects the ac perfor-

mances, 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 impedance and the maximum input frequency as shown

in Figure 8.

FREQUENCY – kHz

–110

100

–100

–90

–80

–70

1000

R = 100

R = 50

R = 11

Figure 8. THD vs. Analog Input Frequency and Input

Resistance (0 V to 2.5 V Only)

REV. C

375 Ω

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