AD628ARM datasheet(16/20 Pages) AD

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AD628

Rev. C | Page 16 of 20

The differential input voltage range is constrained to the

linear operation of the internal amplifiers A1 and A2. The

voltage applied to the inputs of A1 and A2 should be between

VS− + 1.2 V and VS+ − 1.2 V. Similarly, the outputs of A1 and A2

should be kept between VS− + 0.9 V and VS+ − 0.9 V.

VOLTAGE LEVEL CONVERSION

Industrial signal conditioning and control applications typically

require connections between remote sensors or amplifiers and

centrally located control modules. Signal conditioners provide

output voltages up to ±10 V full scale; however, ADCs or

microprocessors operating on single 3.3 V to 5 V logic supplies

are becoming the norm. Thus, the controller voltages require

further reduction in amplitude and reference.

Furthermore, voltage potentials between locations are seldom

compatible, and power line peaks and surges can generate

destructive energy between utility grids. The AD628 is an ideal

solution to both problems. It attenuates otherwise destructive

signal voltage peaks and surges by a factor of 10 and shifts the

differential input signal to the desired output voltage.

Conversion from voltage-driven or current-loop systems is

easily accommodated using the circuit in Figure 32. This shows

a circuit for converting inputs of various polarities and

amplitudes to the input of a single-supply ADC.

Note that the common-mode output voltage can be adjusted by

connecting Pin 3 (VREF) and the lower end of the 10 kΩ resistor

to the desired voltage. The output common-mode voltage will

be the same as the reference voltage.

The design of such an application may be done in a few simple

steps, which include the following:

• Determine the required gain. For example, if the input voltage

must be transformed from ±10 V to 0 V to +5 V, the gain is

+5/+20 or +0.25.

• Determine if the circuit common-mode voltage must be

changed. An AD7715-5 ADC is illustrated for this example.

When operating from a 5 V supply, the common-mode

voltage of the AD7715 is half the supply or 2.5 V. If the

AD628 reference pin and the lower terminal of the 10 kΩ

resistor are connected to a 2.5 V voltage source, the output

common-mode voltage will be 2.5 V.

Table 6 shows resistor and reference values for commonly used

single-supply converter voltages. REXT3 is included as an option.

It is used to balance the source impedance into A2, which is

described in more detail in the Gain Adjustment section.

Table 6. Nearest 1% Resistor Values for Voltages Level

Conversion Applications

Input

Voltage

(V)

ADC

Supply

Voltage

(V)

Desired

Output

Voltage (V)

VREF

(V)

REXT1

(kΩ)

REXT3

(kΩ)

±10

2.5

15.0

4.02

±5

2.5

39.7

2.00

+10

2.5

39.7

2.00

2.5

89.8

1.00

±10

1.25

2.49

7.96

±5

1.25

15.0

4.02

+10

1.25

15.0

4.02

1.25

39.7

2.00

100k

Ω

–IN

AD628

+VS

+IN

–VS

VREF

+IN

–IN

100k

Ω

10k

Ω

CFILT

VIN

(SEE

TABLE 5)

10k

Ω

10k

Ω

+IN

–IN

REXT3

(SEE

TABLE 5)

REXT1

OUT

10k

Ω

G = +0.1

(SEE

TABLE 5)

+5V

AD680

+5V

AD7715-5

SERIAL CLOCK

CLOCK

+2.5V

SCLK

MCLK IN

MCLK OUT

RESET

AVDD

AIN(+)

AIN(–)

DGND

DVDD

DIN

DOUT

DRDY

AGND

REF IN(–)

REF IN(+)

Figure 32. Level Shifter

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