AD628
Rev. G | Page 17 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
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.
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, which includes the following:
• Determine the required gain. For example, if the input
voltage must be changed from ±10 V to +5 V, the gain now
needs to be +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
Input
Voltage (V)
ADC
Supply
Voltage (V)
Desired
Output
Voltage (V)
V
REF
(V)
R
EXT1
(kΩ)
R
EXT2
kΩ)
±10 5 2.5 2.5 15 10
±5 5 2.5 2.5 39.7 10
+10 5 2.5 0 39.7 10
+5 5 2.5 0 89.8 10
±10 3 1.25 1.25 2.49 10
±5 3 1.25 1.25 15 10
+10 3 1.25 0 15 10
+5 3 1.25 0 39.7 10
5
1
3 4
8
2
7
+V
S
–V
S
6
–IN
+IN
V
REF
100k
10k
100k
10k
10k
A1
A2
4
5
6
3
1
2
SCLK
SDATA
CS
GND
V
DD
V
IN
AD628
SERIAL DATA
REF195
+12V
V
OUT
V
IN
2
3
4
6
C
FILT
R
G
10F
0.1F
10F
0.1F
10F0.1F 10F0.1F
AD7940
±10V
15nF
2
3
1
AD8606
1/2
49.9
33nF
+12
12
10k
10k
AD628 REFERENCE VO LTAGE
R
EXT2
10k
R
E
X
T
1
1
5
k
AD8606
2/2
5
6
7
4
8
02992-030
OUT
Figure 32. Level Shifter
