Figure 11 is a similar application that isolates the digital
outputs of the three ADS7817s instead of the analog signal
from the motor. Here, the reference voltage for the ADS7817
is 150mV, and the analog input of each ADS7817 is con-
nected directly to the current sense resistor. By removing the
ISO130 from the signal path, a greater signal-to-noise ratio is
achieved in the sensing system. However, nine optical isola-
tors are needed to isolate the A/D converters.
APPLICATION CIRCUITS
Figures 9, 10 and 11 show some typical applications circuits
for the ADS7817. Figure 9 shows a low cost, low power
circuit for basic data acquisition. Total power dissipation in
the ADS7817 and reference circuitry is under 5mW over
temperature, power supply variations, and at a 200kHz sample
rate.
Figure 10 is a motor control application using three ISO130s
to isolate the motor from the sensing system (three ADS7817s
and a DSP56004). The ISO130 provides 10kV/µs (minimum)
isolation-mode rejection, 85kHz large signal bandwidth, and
a fixed gain of 8. The ADS7817’s reference voltage is 1.2V
and is derived from a REF1004-1.2. This gives the converter
a full-scale input range of ±1.2V. Because of the gain of 8 in
the ISO130, the current sense resistor should give a worst-
case output voltage of less than ±150mV.
+5V
5Ω to 10Ω
+
1µF to
10µF
24.9kΩ
22Ω
ADS7817
VREF
VCC
CS
1µF to
10µF
+
+
REF1004-2.5
4.7µF
0.1µF
Microcontroller
+In
–In
DOUT
GND
DCLOCK
FIGURE 9. Low Cost, Low Power Data Acquisition System.
®
ADS7817
13
BB [ BURR-BROWN CORPORATION ]
