AN-1208: Programmable Bidirectional Current Source Using the AD5292 Digital Potentiometer and the ADA4091-4 Op Amp (Rev. C) PDF

AN-1208
APPLICATION NOTE
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Programmable Bidirectional Current Source Using the AD5292 Digital
Potentiometer and the ADA4091-4 Op Amp
CIRCUIT FUNCTION AND BENEFITS
CIRCUIT DESCRIPTION
The circuit in Figure 1 provides a programmable bidirectional
Howland current source using the AD5292 digital potentiometer
in conjunction with the quad ADA4091-4 op amp and the
ADR512 voltage reference. This circuit offers 10-bit resolution
over an output current range of ±18.4 mA. The AD5292 is
programmable over an SPI-compatible serial interface.
Table 1. Devices Connected/Referenced
The ±1% resistor tolerance of the AD5292 allows it to be placed
in series with external divider resistors, as shown in Figure 5, to
reduce the maximum output current without the need to match
the resistors in the circuit. Reducing the IOUT range serves to
increase the sensitivity of the output current.
ADR512
Product
AD5292
ADA4091-4
This circuit employs the AD5292 digital potentiometer in
conjunction with the ADR512 reference and the ADA4091-4 op
amp, providing a 10-bit, programmable, bidirectional current
source. The circuit guarantees monotonicity, ±1 LSB DNL, and
has an integral nonlinearity of ±2 LSB, typical.
The AD5292 has an internal 20-times programmable memory
that allows a customized IOUT at power-up. The circuit provides
accurate, low noise, and low tempco output voltage capability
and is well suited for digital calibration applications.
U1A
Description
Digital potentiometer, 10 bits, 1% resistor
tolerance
Micropower, overvoltage protected (OVP),
rail-to-rail op amp
Low noise, precision 1.2 V reference
The bipolar high voltage regulator consists of a low voltage
reference followed by a noninverting and an inverting amplifier
whose gains are set by the ratio of R1 to R2.and R3 to R4. The
ADR512 1.200 V voltage reference has low temperature drift,
high accuracy, and ultralow noise performance.
+V
+15V
1/4
ADA4091-4
V1
+13.8V
VDD
–15V
+15V
–V
VDD
+15V
RBIAS
ADR512
3.9kΩ
+1.2V
R2
R1
RAB 20kΩ
16.5kΩ ± 1%
3/4
1.05kΩ ± 1%
R6B
50Ω ± 1%
C1
10pF
IOUT
R6A
U1D
3/4
RLOAD
ADA4091-4
–15V
VSS
R9
1kΩ
R6
ADA4091-4
SERIAL
INTERFACE
U1B
10nF
VW
10.5kΩ ± 1%
1kΩ ± 1%
U1C
AD5292
2/4
ADA4091-4
–V2
–13.8V
R7
R8
16.5kΩ ± 1%
1.1kΩ ± 1%
1kΩ ± 1%
R4
10pF
11.5kΩ ± 1%
Figure 1. Programmable Bidirectional Current Source (Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. C | Page 1 of 3
08459-001
C2
R3
AN-1208
Application Note
20
(1)
10
IOUT (mA)
As shown in Figure 1, the RBIAS resistor is 3.9 kΩ, which sets the
bias current of the ADR512 at 3.5 mA.
The ADA4091-4 is an op amp that offers low offset voltage and
rail-to-rail output. The ADR512, in combination with the
ADA4091, offers a low tempco and low noise output voltage.
The R1 and R2 resistors adjust the gain in the noninverting
amplifier, U1A. The output voltage, V1, defines the maximum
positive output current range. Equation 2 and Equation 3 are
used to calculate the resistor values.
I OUT
(2)
V1 ≈
1.33 × 10 − 3
0
–10
–20
08459-002
The maximum resistor that ensures an ADR512 minimum
operating current is defined in Equation 1.
V − 1.2 V
RBIAS = DD
1.5 mA
500
0
1000
CODE (Decimal)
Figure 2. Maximum Output Current Versus Decimal Code
Typical INL and DNL plots are shown in Figure 3 and Figure 4.
1.5
(3)
1.0
The maximum negative output current range is adjusted by R3
and R4, which define the output voltage, V2, in the inverting
amplifier, U1B. Equation 4 and Equation 5 are used to calculate
the resistor values.
I OUT
1.33 × 10− 3
V2 = 1.2 × (−
R1
)
R2
INL (LSB)
(4)
–0.5
–1.0
(5)
–1.5
0
The resistors, which are shown in Figure 1, are chosen to
provide a gain of +11.5 and −11.5 in the noninverting and the
inverting amplifier, respectively. This provides a bipolar regulated
voltage of ±13.8 V. These voltages can be used to power other
circuits with a maximum output current of +17 mA.
=
D × (V1 − V2 )
D × 27.6
+ V2 =
− 13.8
1024
1024
600
800
1000
800
1000
Figure 3. INL Versus Decimal Code
0.7
0.2
(6)
–0.3
(1.05 kΩ + 50 Ω) × VW
= 1.33 × 10 −3 × VW
16.5 kΩ × 50 Ω
VW =
400
DNL (LSB)
R6 A + R6 B
× VW =
R7 × R6 B
200
CODE (Decimal)
Equation 6 and Equation 7 calculate the output current of the
Howland current source, and Figure 2 shows the maximum IOUT
versus code.
I OUT =
0
08459-003
V2 ≈
0.5
(7)
08459-004
R
V1 = 1.2 × (1 + 1 )
R2
–0.8
0
200
400
600
CODE (Decimal)
where D is the code loaded in the digital potentiometer.
R6 A + R6 B = R8
(8)
R5 = R7
(9)
Figure 4. DNL Versus Decimal Code
As shown in Figure 1, the bidirectional current source operates
over the maximum output range of ±18.4 mA. To improve the
circuit accuracy the maximum output current, IOUT, should be
decreased by recalculating the resistor value in the U1C and
U1D op amps or by reducing the voltage reference across the
AD5292. This gives the full 10-bit resolution over a limited
output current range.
Rev. C | Page 2 of 3
Application Note
AN-1208
The U1C and U1D op amp resistors can be recalculated using
Equation 6 and Equation 7, but care should be taken to
minimize errors when selecting standard resistor values from
the calculated values. Decreasing the reference voltages, V1 and
V2, in the AD5292 can be accomplished by recalculating the
bipolar output regulator and the U1A and U1B output voltages
or by using two external resistors, as shown in Figure 5.
V1
+13.8V
R10
17.8kΩ ± 1%
V'1
+4.96V
VW
V'2
–4.96V
R11
17.8kΩ ± 1%
V2
–13.8V
The ADA4091-2 dual op amp can be used when the voltage
references, V1 and V2, are not necessary.
MT-031 Tutorial, Grounding Data Converters and Solving
the Mystery of "AGND" and "DGND," Analog Devices.
MT-032 Tutorial, Ideal Voltage Feedback (VFB) Op Amp,
Analog Devices.
MT-087 Tutorial, Voltage References, Analog Devices.
08459-005
20kΩ
The AD5291 (eight bits with 20-times programmable power-up
memory) and the AD5293 (10 bits, no power-up memory) are
both ±1% tolerance digital potentiometers that are suitable for
this application.
LEARN MORE
AD5292
SERIAL
INTERFACE
COMMON VARIATIONS
MT-091 Tutorial, Digital Potentiometers, Analog Devices.
MT-095 Tutorial, EMI, RFI, and Shielding Concepts,
Analog Devices.
Figure 5. Improvement in Accuracy Using Reduced Reference Voltages
(Simplified Schematic: Decoupling and All Connections Not Shown)
The resistors in series with the AD5292 are useful when the
voltage references, V1 and V2, are the main system power
supplies. Traditionally, digital potentiometers have a ±20%
end-to-end resistor tolerance error. This affects the circuit
accuracy because of the mismatch error between the digital
potentiometer and the external resistors. The industry leading
±1% resistor tolerance performance of the AD5292 helps to
overcome the mismatch resistance error.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
Data Sheets
AD5292 Data Sheet
AD5291 Data Sheet
AD5293 Data Sheet
ADR512 Data Sheet
The AD5292 has 20-times programmable memory, which
allows presetting the circuit output current to a specific value
at power-up.
ADA4091-2 Data Sheet
Excellent layout, grounding, and decoupling techniques must
be used to achieve the desired performance from the circuits
discussed in this note (see MT-031 Tutorial and MT-101 Tutorial).
As a minimum, a 4-layer PCB should be used with one ground
plane layer, one power plane layer, and two signal layers.
REVISION HISTORY
ADA4091-4 Data Sheet
4/13—Rev. B to Rev. C
Changed Document Title from CN-0177 to
AN-1208 .............................................................................. Universal
3/11—Rev. A to Rev. B
Change to Figure 1 ............................................................................ 1
3/10—Rev. 0 to Rev. A
Changes to Circuit Function and Benefits Section....................... 1
9/09—Revision 0: Initial Version
©2009–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN08459-0-4/13(C)
Rev. C | Page 3 of 3
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