AD628
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.
Designing such an application can be done in a few simple
steps, including 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 should be
changed. An AD7940 ADC is illustrated for this example.
When operating from a 5 V supply, the common-mode
voltage of the AD7940 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 is 2.5 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 of up to 10 V full scale. However, ADCs or
microprocessors operating on single 3.3 V to 5 V logic supplies
are now the norm. Thus, the controller voltages require further
reduction in amplitude and reference.
Table 6 shows resistor and reference values for commonly used
single-supply converter voltages. REXT3 is included as an option
to balance the source impedance into A2. This is described in
more detail in the Gain Adjustment section.
Furthermore, voltage potentials between locations are seldom
compatible, and power line peaks and surges can generate
destructive energy between utility grids. The AD628 offers an
ideal solution to both problems. It attenuates otherwise destruc-
tive signal voltage peaks and surges by a factor of 10 and shifts
the differential input signal to the desired output voltage.
Table 6. Nearest 1% Resistor Values for Voltage Level
Conversion Applications
ADC
Supply
Desired
Output
Input
VREF
REXT±
(kΩ)
REXT1
(kΩ)
Voltage (V) Voltage (V) Voltage (V) (V)
Conversion from voltage-driven or current-loop systems is
easily accomplished using the circuit shown in Figure 32. This
shows a circuit for converting inputs of various polarities and
amplitudes to the input of a single-supply ADC.
10
5
5
5
5
5
3
3
3
3
2.5
2.5
2.5
0
15
10
10
10
10
10
10
10
10
2.5
39.7
39.7
89.8
2.49
15
10
2.5
5
2.5
0
To adjust common-mode output voltage, connect Pin 3 (VREF
)
10
5
1.25
1.25
1.25
1.25
1.25
1.25
0
and the lower end of the 10 kΩ resistor to the desired voltage.
The output common-mode voltage is the same as the reference
voltage.
10
5
15
0
39.7
Rev. F | Page 16 of 20
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