Texas Instruments | Low-level voltage-to-current converter circuit | Application notes | Texas Instruments Low-level voltage-to-current converter circuit Application notes

Texas Instruments Low-level voltage-to-current converter circuit Application notes
Analog Engineer's Circuit: Amplifiers
SBOA233 – January 2019
Low-level voltage-to-current converter circuit
Design Goals
Input
Output
Supply
Load Resistance (RL)
ViMin
ViMax
ILMin
ILMax
Vcc
Vee
RLMin
RLMax
0.49V
4.9V
1µA
10µA
5V
0V
0Ω
390kΩ
Design Description
This circuit delivers a precise low-level current, IL, to a load, RL. The design operates on a single 5-V
supply and uses one precision low-drift op amp and one instrumentation amplifier. Simple modifications
can change the range and accuracy of the voltage-to-current (V-I) converter.
R3 100k
VoOPA
+5V
-
R2 200k
R4 100k
C2 510p
C1 10n
+
+5V
U1 OPA333
+
V
Rset 100k
Vi 4.9
Vset
R1 81.6k
+
+
R1 +
R2
VoINA
R1
-
U2 INA326
-
INAgain = 4.9
VLoad
RL 390k
+5V
+
A
IL
Vcc 5
Design Notes
1. Voltage compliance is dominated by op amp linear output swing (see data sheet AOL test conditions)
and instrumentation amplifier linear output swing. See the Common-Mode Input Range Calculator for
Instrumentation Amplifiers for more information.
2. Voltage compliance, along with RLMin, RLMax, and Rset bound the IL range.
3. Check op amp and instrumentation amplifier input common-mode voltage range.
4. Stability analysis must be done to choose R4 and C1 for stable operation.
5. Loop stability analysis to select R4 and C1 will be different for each design. The compensation shown is
only valid for the resistive load ranges used in this design. Other types of loads, op amps, or
instrumentation amplifiers, or both will require different compensation. See the Design References
section for more op amp stability resources.
SBOA233 – January 2019
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Low-level voltage-to-current converter circuit
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Design Steps
1. Select Rset and check ILMin based on voltage compliance.
R3
C1
R4
4.9V
+5V
0.100V
-
R2
C2
VoOPA
4.925V
+
Vi
U1 OPA333
R1 +
+
+5V
R2
V
Rset
R1
+
+
Vset
VoINA
R1
U2 INA326
-
-
0.075V
Voltage Compliance Considerations
VLoad
RL
+5V
+
A
Vcc 5
IL
2. Compute instrumentation amplifier gain, G.
3. Choose R1 for INA326 instrumentation amplifier gain, G. Use data sheet recommended R2 = 200kΩ
and C2 = 510pF.
4. The final transfer function of the circuit follows:
2
Low-level voltage-to-current converter circuit
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Design Simulations
DC Simulation Results
Vi
T
VoOPA
VoOPA
Compliance
VoINA
VoINA
Compliance
0.999627µA
99.982723mV
100mV to 4.9V
490.013346mV
75mV to 4.925V
0.999627µA
489.837228mV
100mV to 4.9V
490.013233mV
75mV to 4.925V
0Ω
9.996034µA
999.623352mV
100mV to 4.9V
4.900016V
75mV to 4.925V
390kΩ
9.996031µA
4.898075V
100mV to 4.9V
4.900015V
75mV to 4.925V
RL
IL
0.49V
0Ω
0.49V
390kΩ
4.9V
4.9V
10u
9u
8u
7u
6u
IL (A) 5u
4u
3u
2u
1u
0
0.0
500.0m 1.0
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1.5
2.0
2.5
Vi (V)
3.0
3.5
4.0
4.5
5.0
Low-level voltage-to-current converter circuit
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Design References
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
See the TINA-TI™ circuit simulation file, SBOMAT8.
See TIPD107, http://www.ti.com/tool/TIPD107.
See Solving Op Amp Stability Issues - E2E FAQ.
See TI Precision Labs - Op Amps.
Design Featured Op Amp
OPA333
Vss
1.8V to 5.5V
VinCM
Rail-to-rail
Vout
Rail-to-rail
Vos
2µV
Iq
17µA/Ch
Ib
70pA
UGBW
350kHz
SR
0.16V/µs
#Channels
1,2
http://www.ti.com/product/opa333
Design Featured Instrumentation Amplifier
INA326
Vss
2.7V to 5.5V
VinCM
Rail-to-rail
Vout
Rail-to-rail
Vos
20µV
Iq
2.4mA
Ib
0.2nA
UGBW
1kHz (set by 1kHz filter)
SR
0.012V/µs (set by 1kHz filter)
#Channels
1
http://www.ti.com/product/INA326
4
Low-level voltage-to-current converter circuit
Copyright © 2019, Texas Instruments Incorporated
SBOA233 – January 2019
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