Digitizing

APPLICATION NOTE 105: Current Sense Circuit Collection
Digitizing
To see other chapters in this Application Note, return to
the Introduction.
In many systems the analog voltage quantity indicating
current flow must be input to a system controller. In this
chapter several examples of the direct interface of a current sense amplifier to an A to D converter are shown.
Sensing Output Current
VCC
0V TO 1V
12V
VCSRC
VCSNK
+IN
–IN
EN
VCC
V+
ISRC
ISNK
RS
0.2Ω
TSD
OUT
LT1970
SENSE+
SENSE–
FILTER
V–
VEE
RLOAD
COMMON
R4
255k
LT1787
RG
RF
VS–
–12V
VS+
20k
VEE
–12V
BIAS
–12V
R1
60.4k
R2
10k
12V
–
+
VOUT
2.5V
±5mV/mA
LT1880
R3
20k
1kHz FULL CURRENT
BANDWIDTH
–12V
0V TO 5V
A/D
1970 F10
OPTIONAL DIGITAL FEEDBACK
The LT1970 is a 500mA power amplifier with voltage
programmable output current limit. Separate DC voltage
inputs and an output current sensing resistor control the
maximum sourcing and sinking current values. These
control voltages could be provided by a D-to-A Converter
in a microprocessor controlled system. For closed loop
control of the current to a load an LT1787 can monitor
the output current. The LT1880 op amp provides scaling
and level shifting of the voltage applied to an A-to-D
Converter for a 5mV/mA feedback signal.
Digitizing-1
APPLICATION NOTE 105: Current Sense Circuit Collection
Split or Single Supply Operation, Bidirectional Output into A/D
1Ω
1%
IS = ±125mA
VSRCE
≈4.75V
1
FIL–
–
2 VS
3
DNC
VEE 4
VEE
–5V
VCC
5V
8
FIL+
LT1787
VS+ 7
10µF
16V
VBIAS 6
20k
VOUT
5
1
CONV
VOUT (±1V)
7
2
6
AIN LTC1404 CLK
VREF
5
DOUT
GND
10µF
16V
4
8
3
OPTIONAL SINGLE
SUPPLY OPERATION:
DISCONNECT VBIAS
FROM GROUND
AND CONNECT IT TO VREF.
REPLACE –5V SUPPLY
WITH GROUND.
OUTPUT CODE FOR ZERO
CURRENT WILL BE ~2430
CLOCKING
CIRCUITRY
10µF
16V
DOUT
VEE
–5V
1787 TA02
In this circuit, split supply operation is used on both the
LT1787 and LT1404 to provide a symmetric bidirectional
measurement. In the single-supply case, where the
LT1787 pin 6 is driven by VREF, the bidirectional measurement range is slightly asymmetric due to VREF being
somewhat greater than mid-span of the ADC input range.
16-Bit Resolution Unidirectional
Output into LTC2433 ADC
12-Bit Resolution Unidirectional
Output into LTC1286 ADC
TO
LOAD
I = 100A
1
FIL–
2 VS
3
4
RSENSE
0.0016Ω
–
DNC
FIL+
LT1787HV
ROUT
20k
VEE
Digitizing-2
8
2.5V TO 60V
7
R1
15k
VBIAS 6
5
VOUT
VOUT = VBIAS + (8 • ILOAD • RSENSE)
The LTC2433-1 can accurately digitize signal with source
impedances up to 5kΩ. This LTC6101 current sense circuit uses a 4.99kΩ output resistance to meet this requirement, thus no additional buffering is necessary.
VS
+
C2
0.1µF
C1
1µF
VREF VCC
CS
+IN
LTC1286 CLK
–IN
D
GND OUT
LT1634-1.25
5V
TO µP
1787 TA01
While the LT1787 is able to provide a bidirectional output, in this application the economical LTC1286 is used
to digitize a unidirectional measurement. The LT1787 has
a nominal gain of eight, providing a 1.25V full-scale output at approximately 100A of load current.