AD7750

–7–

REV. 0

ANALOG INPUTS

The analog inputs of the AD7750 are high impedance bipolar

voltage inputs. The four voltage inputs make up two truly

any ADC, an antialiasing filter or low-pass filter is required on

the analog input. The AD7750 is designed with a unique

switched capacitor architecture that allows a bipolar analog

input with a single 5 V power supply. The four analog inputs

+1.0 V. This is an absolute voltage range and is relative to the

ground (AGND) pin. This ground is nominally at a potential of

0 V relative to the board level ground. Figure 8 shows a very

simplified diagram of the analog input structure. When the ana-

log input voltage is sampled, the switch is closed and a very

small sampling capacitor is charged up to the input voltage. The

resistor in the diagram can be thought of as a lumped compo-

nent made up of the on resistance of various switches.

R

1.4k

C

2pF

SAMPLING

CAPACITOR

Figure 8. Equivalent Analog Input Circuit

Analog Inputs Protection Circuitry

The analog input section also has protection circuitry. Since the

power supply rails are 0 V to 5 V, the analog inputs can no

longer be clamped to the supply rails by diodes. Thus, the inter-

nal protection circuitry monitors the current paths during a fault

condition and protects the device from continuous overvoltage,

continuous undervoltage and ESD events. The maximum over-

voltage the AD7750 analog inputs can withstand without caus-

ing irreversible damage is

±6 V relative to AGND pin.

In the case of continuous overvoltage and undervoltage the

series resistance of the antialiasing filter can be used to limit

input current. The total input current in the case of a fault

should be limited to 10 mA.

For normal operation of the AD7750 there are two further re-

strictions on the signal levels presented to the analog inputs.

1. The voltage on any input relative to the AGND pin must not

exceed

±1 V.

2. The differential voltage presented to the ADC (Analog

Modulator) must not exceed

±2 V.

±1 V. These signals are not gained (G1 = 0) and so the

differential signal presented to the modulator is

±2 V.

However, Channel 2 has an associated gain of two and so

care must be taken to ensure the modulator input does not

exceed

±2 V. Therefore, the maximum signal voltage that

±0.5 V.

The difference between single-ended and complementary

differential input schemes is shown in the diagram below,

Figure 9. For a single-ended input scheme the V– input is

held at the same potential as the AGND Pin. The maxi-

mum voltages can then be applied to the V+ input are

shown in Figures 10 and 11. An example of this input

scheme uses a shunt resistor to convert the line current to a

An example of the complementary differential input scheme

uses a current transformer to convert the line current to a

scheme the voltage on the V+ input is always equal to, but

of opposite polarity to the voltage on V–. The maximum

voltage that can be applied to the inputs of the AD7750

using this scheme is shown in Figures 12 and 13.

Note that the common mode of the analog inputs must

be driven. The output terminals of the CT are, therefore,

referenced to ground.

A CURRENT TRANSFORMER

PROVIDES COMPLEMENTARY

DIFFERENTIAL INPUTS TO THE

AD7750

A CURRENT SENSE RESISTOR

PROVIDES A SINGLE-ENDED

INPUT TO THE AD7750

V+

V–

V–

V+

SHUNT

RESISTOR

Figure 9. Examples of Complementary and Single-

Ended Input Schemes