AD628
CURRENT LOOP RECEIVER
MONITORING BATTERY VOLTAGES
Analog data transmitted on a 4 to 20 mA current loop can be
detected with the receiver shown in Figure 33. The AD628 is an
ideal choice for such a function because the current loop is
driven with a compliance voltage sufficient to stabilize the loop,
and the resultant common-mode voltage often exceeds commonly
used supply voltages. Note that with large shunt values, a resistance
of equal value must be inserted in series with the inverting
input to compensate for an error at the noninverting input.
Figure 34 illustrates how the AD628 is used to monitor a battery
charger. Voltages approximately eight times the power supply
voltage can be applied to the input with no damage. The resistor
divider action is well suited for the measurement of many
power supply applications, such as those found in battery
chargers or similar equipment.
For proper operation, the common-mode voltage must satisfy
the input specifications in Table 1, as well as Equation 2.
V
= 15V
+15V –15V
CM
3
7
2
4
AD628
10kꢀ
249ꢀ
100kꢀ
1
8
10kꢀ
0V TO 5V
TO ADC
5
249ꢀ
100kꢀ
10kꢀ
6
I = 4 TO 20mA
210kꢀ
100kꢀ
+2.5V
9.53kꢀ
Figure 33. Level Shifter for 4 to 20 mA Current Loop
+5V
nV
(V)
–IN
BAT
100kꢀ
100kꢀ
10kꢀ
10kꢀ
+IN
–IN
OUT
TO ADC
A2
G = +0.1
A1
R
–IN
+IN
EXT1
10kꢀ
CHARGING
CIRCUIT
+1.5V
BATTERY
R
G
+IN
OTHER
BATTERIES IN
CHARGING
CIRCUIT
10kꢀ
AD628
V
C
FILT
REF
–5V
Figure 34. Battery Voltage Monitor
Rev. G | Page 18 of 20
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