AN-968: Current Sources: Options and Circuits (Rev. 0) PDF

AN-968
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Current Sources: Options and Circuits
by Martin Murnane
VDD
REFIN1(+) GND
IN+
OUT–
OUT+
AVDD
AD7794/AD7795
AIN1(+)
AIN1(–)
IN+
OUT–
VDD
IN–
OUT+
AIN2(+)
DOUT/RDY
AIN2(–)
IN–
AIN3(+)
MUX
BUF
IN-AMP
Σ-Δ
ADC
SERIAL
INTERFACE
AND
LOGIC
CONTROL
DIN
SCLK
CS
AIN3(–)
GND
REFIN2(+)
RCM
VDD
REFIN2(–)
IOUT1
INTERNAL
CLOCK
DVDD
REFIN1(–)
GND
CLK
07488-001
PSW
Figure 1. AD7794 Current Source Application (See AD7794 Data Sheet for More Details)
INTRODUCTION
LOW CURRENT—ADC APPLICATION
Many applications require current sources to excite various devices
for sensor drive, accurate measurement, and other applications.
This application note discusses several options that are available
to designers when designing current sources using ICs from
Analog Devices, Inc. It shows examples of current sources from
the microampere range which are integrated in specific devices
and also medium to high power discrete applications up to the
1 A range.
Some ADCs are designed specifically for direct sensor
attachment with internal constant current sources, also
called excitation currents.
The AD7794 device has two programmable excitation currents
that can be programmed from 10 μA to 1 mA (see Figure 1).
The current sources are controlled via a register (I/O register)
that enables and directs the current to one of two output pins,
in this case IOUT1 (see Figure 1 for details). This is adequate
for low power portable applications where sensor power
consumption must be low.
Rev. 0 | Page 1 of 4
AN-968
Application Note
LOW CURRENT—OP AMP APPLICATION
A discrete option for a current source is an op amp driven circuit
is shown in Figure 3. The AD8610 op amp is a relatively high
current drive op amp, driven from ±12 V.
10pF
2kΩ
VIN
There are many other ADCs with a constant current source
function available from Analog Devices. For additional information, see www.analog.com/adcs.
LK1
2kΩ
2
22Ω
6
3
AD8610ARZ
1MΩ
5V
LOAD
The AD7719 ADC has a similar current source setup, with
the maximum current at 400 μA (see Figure 2). Similarly, two
current sources are available. Both current sources are 200 μA
and can be controlled in a similar manner to the AD7794 ADC:
one or both currents can be sent to the one output pin. This is
clearly shown in Figure 2 where both currents are directed to
the IOUT1 pin, which then drives the bridge and the reference.
See the AD7719 data sheet for more details.
1kΩ
10pF
AVDD
IOUT1
2kΩ
I1
10kΩ
I2
AD8610ARZ
Figure 3. 10 mA Current Source/Sink
REFIN(+)
6.25kΩ
REFIN(–)
AIN2
IN(+)
XTAL1
AD7719
OUT(+)
OUT(–)
PRESSURE
BRIDGE
AIN1
XTAL2
AIN4
IN(–)
AGND
07488-002
AIN3
250Ω
07488-003
1Ω
Figure 2. AD7719 Current Source Application (see the AD7719 Data Sheet
for More Details)
MICROCONTROLLERS
Analog Devices also offers a range of microcontrollers that
contain current sources in the low current range. For additional
information, see www.analog.com/microcontrollers.
A negative voltage at VIN controls the op amp and raises the
voltage at the output of this device. The output current of the
amplifier flows in the 1 Ω sense resistor. The voltage at the input
of the feedback op amp increases, which produces a voltage of
opposite polarity at the input of the control op amp. A state of
equilibrium is reached and a steady state current flows in the
1 Ω sense resistor. A sense resistor is used to measure current,
but a load resistor can also be used to reduce the cost of expensive
sense resistors. One disadvantage of this is the unknown state
of the circuit if the load is removed, for instance, the amplifier
could saturate.
Note that the AD8610 is chosen because it has excellent current
noise and voltage noise performance. See the AD8610 data
sheet for more information. This circuit can sink or source
10 mA (or greater) by applying a positive or negative voltage
to VIN, respectively.
Rev. 0 | Page 2 of 4
Application Note
AN-968
MEDIUM CURRENT—BIPOLAR APPLICATION
HIGH CURRENT—MOSFET APPLICATION
An example of a current source that has an even larger current
supply (100 mA or greater) is in the circuit shown in Figure 4.
This circuit uses an op amp output driver push-pull stage to
supply current to the load. When a positive voltage is applied to
VIN, the output voltage of the control op amp rises, which turns
on Q1 and drives current through the 10 Ω resistor to the load.
The 10 Ω resistors are required to prevent thermal runaway.
As the current rises, the voltage on the sense resistor also rises
and the voltage feedback to the control op amp increases until
a state of equilibrium is reached. On reaching equilibrium, a
constant current flows through the load for a fixed input voltage at VIN. This is a constant current source as it is sourcing
current to the load.
If a higher current application is required, then the previous
circuit can be adjusted to increase the current in the load by
replacing the push-pull with a MOSFET and a few other
components.
EXT
+5V
10Ω
2
Q1
R3
10Ω
E
B
22pF
12kΩ
Q2
C
0.1µF
–12V
10µF
1Ω
SENSE
RESISTOR
07488-005
This circuit in Figure 5 uses a control loop to set the gate voltage
of a MOSFET (IRF640 N-channel). The circuit in Figure 5 uses
a sense resistor and a feedback amplifier to reduce the sensitivity
of VIN, as mentioned in the previous example. The maximum
current of Figure 5 is 1000 mA. However, the same control loop
can be used to drive even higher currents by changing only the
MOSFET and the sense resistor. Also, an advantage of the circuit
in Figure 5 is that a different voltage supply can be used on the
load rather than the supply powering the circuit, as indicated by
Jumper LK2. This means that if a high voltage MOSFET is used,
like the IRF640, which has a 200 V absolute maximum rating,
then this circuit can operate with much higher voltages than
that of the 15 V powering the rest of the circuit.
E
R2
10Ω
D1
10kΩ
Figure 5. Current Sink Using IRF640 MOSFET, 1000 mA
C
D2
1
AD8610ARZ
100mΩ
LOAD
2kΩ
22pF
SENSE RESISTOR
15kΩ
07488-004
LK1
56Ω
190kΩ
10µF
P4
1
2
6
0.1µF
10MΩ
3 U1
3
LK1
5 15kΩ
+12V
VIN
10kΩ
7
Adding an amplifier at the feedback loop decreases the sensitivity at VIN. For example, a gain of 10 allows a control voltage
at VIN of ±1 V.
B
AD8610ARZ
VIN
For a negative input voltage at VIN, a similar situation arises
except that the Q2 is turned on which causes the current to
flow in the opposite direction. For a constant voltage at VIN,
a constant current flows in the load. With a step input voltage
at VIN, or a VIN voltage of ±100 mV at a frequency of 20 kHz,
the circuit can operate quite well with a current settling time of
~3 μs. This switching gives a good indication of the stability of
the circuit.
12kΩ
LK2
10MΩ
Figure 4. Current Source/Sink, 100 mA or Greater
Rev. 0 | Page 3 of 4
AN-968
Application Note
LAYOUT MODULE
EXT
+5V
+10V
0.1µF
C
12kΩ
LK2
B
Q1
AD8610ARZ
VIN
10MΩ
2
LK1 15kΩ
P4
D2
1
56Ω
3
P4
VIN
E
R2
10Ω
0Ω
R3
10Ω
E
D1
LK3 15kΩ
B
22pF
Q2
12kΩ
15kΩ
LOAD
10µF
+
The circuit is designed as follows: the MOSFET and sense resistor
are selected to sink 1000 mA. Thus, with a sense resistor of
100 mΩ, the total voltage at full load is 0.1 V. The sense resistor
dissipates 0.1 W. The feedback circuit has a gain of 20 so the
total voltage feedback to the control amplifier is 2.0 V. Therefore, the voltage at VIN, required to sink 1000 mA in the load
is 2.0 V. See Figure 6 for the simulation response. To control this
voltage by driving VIN with a DAC varies the current in the
load, thus making this a variable current source. Fixing the
voltage at VIN to 1.0 V produces a constant current source
of 500 mA.
C
0.1µF
5 15kΩ
7
AD8610ARZ
6
200mΩ
190kΩ
10kΩ
Figure 7. Current Sink Circuit for Full Layout Module
07488-006
VOLTAGE (VIN)
2V/DIV
Figure 6. Simulated Response of the Circuit to a Step Response of the Current
Through Sense Resistor (Duty Cycle is 25%)
The load can be placed at the drain, the source of the MOSFET,
or anywhere in the current path, for operation.
Since heat dissipation of the MOSFET is also critical, a very important factor in determining a MOSFET is its RDS(ON) value. In this
case the RDS(ON) value is 150 mΩ typically. For larger currents,
consider using a RDS(ON) value of <20 mΩ, if possible.
To develop a layout module for a current source that results
in a module that can be used across a wide range of current
source applications, use the circuit in Figure 7 from <1 mA to
>1000 mA. Thus, depending on the current range required, the
same module can be used but only those components required
need to be placed in the PCB.
CONCLUSION
Stability in a constant or variable current source is critical for
accurate measurements. Analog Devices offers a range of
devices that can produce flexible and reliable current sources,
either integrated as in Figure 1 and Figure 2 or discrete as in
Figure 3, Figure 4, Figure 5, and Figure 7 for a wide range of
applications.
©2008 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN07488-0-10/08(0)
Rev. 0 | Page 4 of 4
07488-007
10µF
CURRENT SINK
1A/DIV
SENSE RESISTOR
+
–10V
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