
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  
V
S
 + 1.2 V and V
S+
  1.2 V. Similarly, the outputs of A1 and A2 
should be kept between V
S
 + 0.9 V and V
S+
  0.9 V.  
Rev. F | Page 16 of 20 
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. 
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. 
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. 
To adjust common-mode output voltage, connect Pin 3 (V
REF
) 
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. 
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. 
Table 6 shows resistor and reference values for commonly used 
single-supply converter voltages. R
EXT3
 is included as an option 
to balance the source impedance into A2. This is described in 
more detail in the Gain Adjustment section.  
Table 6. Nearest 1% Resistor Values for Voltage Level 
Conversion Applications 
ADC 
Supply 
Voltage (V)
Voltage (V) 
±10 
5 
2.5 
±5 
5 
2.5 
10 
5 
2.5 
5 
5 
2.5 
±10 
3 
1.25 
±5 
3 
1.25 
10 
3 
1.25 
5 
3 
1.25 
Input 
Voltage (V)
Desired 
Output 
V
REF
(V) 
2.5 
2.5 
0 
0 
1.25 
1.25 
0 
0 
R
EXT1
(kΩ) 
15 
39.7 
39.7 
89.8 
2.49 
15 
15 
39.7 
R
EXT2
(kΩ) 
10 
10 
10 
10 
10 
10 
10 
10