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AD7734

CIRCUIT DESCRIPTION

The AD7734 is a sigma-delta ADC that is intended for the

measurement of wide dynamic range, low frequency signals in

industrial process control, instrumentation, and PLC systems.

It contains thin film resistor dividers, a multiplexer, an input

buffer, a sigma-delta (or charge balancing) ADC, a digital filter,

a clock oscillator, a digital I/O port, and a serial

communications interface.

Analog Front End

The AD7734 features four single-ended analog inputs. The on-

chip thin film resistor dividers allow ±10 V, ±5 V, 0 V to +10 V,

and 0 V to +5 V input signals to be connected directly to the

analog input pins.

The resistor divider input stage is followed by the multiplexer

and then by a wide bandwidth, fast settling time differential

input buffer capable of driving the dynamic load of a high speed

sigma-delta modulator.

In normal circuit configuration, the BIAS0 to BIAS3 and

BIASHI pins are connected to the 2.5 V (reference) voltage

source and the BIASLO pin is connected to 0 V. This ensures

that the differential signal seen by the internal input buffer is

within its absolute/common-mode range of AGND + 200 mV

to AVDD – 300 mV.

The AD7734 AIN voltage should be within the specified

nominal (up to ±10 V) input range, otherwise the performance

on channel might degrade (see the Analog Input’s Extended

Voltage Range section).

If the BIAS pins are in normal configuration, the AIN pin

absolute voltage up to ±16.5 V does not degrade the adjacent

channel’s performance. An AIN absolute voltage over ±16.5 V

results in current flowing through the internal protection

diodes located behind the thin film resistors and the adjacent

channel can be affected.

The AIN pins are “overvoltage tolerant.” However, the absolute

maximum AIN voltage of ±50 V must never be exceeded.

Note that the OVR bit in the channel status register is generated

digitally from the conversion result and indicates the sigma-

delta modulator (nominal) overrange. The OVR bit DOES NOT

indicate exceeding the AIN pin absolute voltage limits.

Figure 23 shows the AD7734 analog input internal structure.

BIAS

AIN

108.5k

Ω

15.5k

Ω

AVDD

MUX

PROTECTION

DIODES

AGND

2.1875V ± 1.25V

2.5V

±10V

BUFFER

Figure 23. Simplified Analog Input Internal Structure

Rev. 0 | Page 26 of 32

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