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Application Note

Analysis of Improved Howland Current Pump
Configurations

Ignacio Vazquez Lam
ABSTRACTThe Improved Howland current pump is a circuit that uses a difference amplifier to impose a voltage across ashunt resistor, creating a voltage-controlled current source capable of driving a wide range of load resistances.
This versatility of this design can be useful in many applications that require a current source capable of bipolar(source or sink) operation. Part of the versatility is the ability to make small alterations to the design that
improves the overall performance of the circuit. This article analyzes a few Improved Howland current pumpconfigurations and provides recommendations on how to enhance performance.
A common goal of these designs is to create a high-output impedance current source that can source or sink
approximately 25 mA of current while employing the Improved Howland current pump topology. Analysis isdone on four different configurations and some benefits and disadvantages of each configuration is discussed.
Depending on design requirements, one configuration can be more appropriate than another for a specificapplication. Take precautions when driving reactive loads in an Improved Howland current pump circuit.
Additionally, some loads can cause the circuit to become unstable due to insufficient phase margin. Only
resistive loads are discussed in this article.

Table of Contents
1 Discrete Improved Howland Current Pump – Design 1.......................................................................................................2
2 Discrete Improved Howland Current Pump With Buffer – Design 2.................................................................................. 43 Integrated Improved Howland Current Pump - INA592 and Buffer – Design 3................................................................. 6
4 Integrated Improved Howland Current Pump - INA592 and Settable Gain – Design 4.....................................................7
5 Design Needs and Considerations........................................................................................................................................8
6 Operational Amplifier Considerations.................................................................................................................................. 9
7 References.............................................................................................................................................................................. 98 Revision History......................................................................................................................................................................9

List of Figures
Figure 1-1. Discrete Improved Howland Current Pump...............................................................................................................2
Figure 1-2. Ideal 10-mA Improved Howland Current Pump.........................................................................................................2
Figure 2-1. Discrete Improved Howland Current Pump With Buffer............................................................................................ 4
Figure 2-2. Ideal 10-mA Improved Howland Current Pump With Buffer ..................................................................................... 5
Figure 3-1. Integrated Improved Howland Current Pump With INA592 and Buffer..................................................................... 6
Figure 4-1. Integrated Improved Howland Current Pump With INA592 and Settable Gain.........................................................7

List of Tables
Table 5-1. Comparison Table for Designs 1-4..............................................................................................................................8
Table 6-1. Operational Amplifier Considerations......................................................................................................................... 9
Table 6-2. Integrated Difference Amplifier Considerations...........................................................................................................9

Trademarks
All trademarks are the property of their respective owners.

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Discrete Improved Howland Current Pump – Design 1

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1 Discrete Improved Howland Current Pump – Design 1
R2
VS-

R1
–
+

R3
Vn

+
+
Vp

Rs
VS+

R4

GND

+

+

V
A

IshuntA

Iload

VshuntA+
GND+

IfeedbackRload
V_termination

Figure 1-1. Discrete Improved Howland Current Pump
Figure 1-1 shows the basic configuration of the Improved Howland current pump that uses one operational
amplifier, five discrete resistors and a resistive load, Rload. The current through the load (Iload) can be calculatedusing Equation 1.

(1)

(2)

In an ideal Improved Howland current pump; resistor R4 is sometimes set to equal R2 - Rs, which produces the
expected current value by slightly altering the feedback in the positive loop. This design has limited practicality
considering the standard resistor values to choose from, as well as their tolerances. More details on thefunctionality of the ideal Improved Howland current pump design can be found in the link provided in the
References section. Figure 1-2 shows an example of Design 1 and the results with a modified R4 resistor. The
circuit is designed for a 10-mA output current with an input voltage difference of 5 V using ideal components.
R2 = 1 k
VS-

R1 = 50 k
R3 = 50 k

+Vn = 0 V+
Vp = 5 V

GND
GND

–
+

Rs = 10  +VS+
R4 = 990 A+
Ifeedback-78.45 A+VAIshunt9.92 mA

Vshunt99.2 mV

+
Iload
10 mA

A

Rload = 100 

GND

Figure 1-2. Ideal 10-mA Improved Howland Current Pump
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Discrete Improved Howland Current Pump – Design 1

Benefits: One benefit of this configuration is the freedom to choose the gain value (G), and ultimately design for
output headroom (the maximum output voltage swing or compliance range), by varying the Vshunt voltage. Thatis the case because all the resistors are discretely selected for the circuit. Another benefit is the ability to selectan op amp which fits the specific design requirements of the application such as size, power, and supply voltage.
One last benefit for this design is that only one op amp is required.
Disadvantages: One disadvantage of this configuration is an error that is caused by the Ifeedback current
affecting the Iload current. An ideal current source has infinite output impedance; however, the finite output
impedance of this configuration is determined by the two feedback resistors in series (R3+R4). This can lead tosignificant error in Iload that is more apparent when the design does not use the modified R4 resistor.
To minimize error caused by Ifeedback, choosing higher value resistors for the feedback paths increases theoutput impedance of the current source. This comes at the expense of more thermal noise due to larger
resistor values. Possible bandwidth limitations and stability issues caused by large resistances and parasitic
capacitances in the circuit also become more prevalent. To learn more about noise and stability, TI Precision Lab
video links are found in the References section.
Another disadvantage with this configuration comes from the discretely chosen resistors in the feedback
network. Discrete builds with 0.1% tolerance resistors can have a worst-case CMRR value of around 60 dB,
which can be too low for precision applications. More information on the importance of matching resistors isfound in the link provided in the References section. This resistor mismatch also creates gain error in the design,
which contributes to the overall error. One final consideration for a discrete version of this configuration is the PC
board space required considering external resistors are being used for the difference amplifier.

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Discrete Improved Howland Current Pump With Buffer – Design 2

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2 Discrete Improved Howland Current Pump With Buffer – Design 2
R2
VS-

R1
–
+

R3
+
+
Vp

Rs
VS+
VS+

GND

A

–

R4

+

+

+

Vn

IfeedbackV
A

IshuntA

Iload

Vshunt+GND+

VSRloadV_termination

Figure 2-1. Discrete Improved Howland Current Pump With Buffer
Figure 2-1 shows a similar configuration of an Improved Howland current pump that uses two op amps. The
buffer has high input impedance, which introduces high output impedance into the current source. Note when thebuffer is added, the circuit designer should no longer modify R4 by the value of Rs. Iload can now be calculatedusing Equation 3 provided below:

(3)

(4)

Benefits: This configuration has the same benefits as the non-buffered configuration shown in Discrete
Improved Howland Current Pump – Design 1; however, it has the added benefit of minimizing error by practicallyeliminating Ifeedback current due to the added buffer. The second op amp therefore results in the ability tochoose lower value resistors for the feedback network. This allows the circuit designer to minimize thermal noise
attributed to high value resistors and also minimizes any stability and bandwidth concerns in the circuit.

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Discrete Improved Howland Current Pump With Buffer – Design 2

Figure 2-2 shows the same 10-mA current source; the buffer practically eliminates Ifeedback current.
R2 = 100 VS-

R1 = 5 k
R3 = 5 k

+
Vn = 0 V+
Vp = 5 V–+

Rs = 10  +

VS+

+

V

R4 = 100 
+

A

–

GND

Ishunt
10 mA

A

Ifeedback0A

Vshunt
100 mV

+
GND

+
Iload
10 mA

A

VS-

Rload = 100 

GND

Figure 2-2. Ideal 10-mA Improved Howland Current Pump With Buffer
Disadvantages: A similar disadvantage to the one op amp design comes from the mismatched discrete
resistors. The overall size of the circuit increases with the addition of a second op amp which can be adisadvantage for designs that are limited in space. Fortunately, many precision op amps are available in dual
configurations, which hardly add to the size or cost of the circuit.

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Integrated Improved Howland Current Pump - INA592 and Buffer – Design 3

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3 Integrated Improved Howland Current Pump - INA592 and Buffer – Design 3
VS+
VCC
IN-

12 k

6 k

INA592
SENSE

+
Vn
–

OUT

+
GND
6 k

+

Rs
REF
VS+

VEE

+

+

Vp

A

–

12 k

+

IN+

IfeedbackV
A

IshuntA

Iload

Vshunt

+

VS+
GND

+

VSRloadV_termination

Figure 3-1. Integrated Improved Howland Current Pump With INA592 and Buffer
Figure 3-1 shows a more integrated version of the Discrete Improved Howland Current Pump With Buffer –Design 2 that integrates the difference amplifier configuration into one package. For a design target of up to ± 25
mA, the INA592 can be a great selection for an integrated design. The performance of this device is attributed
to the core OPA192 precision op amp and precision matched thin-film resistors all integrated into one die. Loadcurrent can accurately be represented by Equation 3; however G is fixed to ½ or 2 (V/V) due to the integrated
resistors. Improved Howland current pump circuits often use a voltage gain of less than 1 (V/V) so the gain of ½
(V/V) is more likely to be of use.
Benefits: The benefit of using Integrated Improved Howland Current Pump - INA592 and Buffer – Design 3
is that it minimizes many of the sources of error seen in the discrete designs. The buffer creates high output
impedance practically eliminating Ifeedback current. The integrated resistors nearly eliminate the error previously
caused by mismatched resistors. As a result, this device has a typical CMRR value of 100 dB as well as a typicalgain error of 0.01%, sufficient for use in high precision applications. Buying discrete precision matched resistors
at this performance level would be a significant expense. Considering the INA592 is priced similarly to other highperformance op amps that do not include the four high precision resistors, the discrete resistors alone can easily
end up costing much more than the INA592 itself.
The 12-kΩ and 6-kΩ integrated resistors also keep thermal noise relatively low. This also minimizes possible
bandwidth limitations and stability issues. Another benefit of the integrated design is the size of the circuit. The
INA592 is offered in a 3-mm × 3-mm VSSOP package, which is significantly smaller than most discrete op amps
paired with four discrete resistors.
For precision applications it is easy to see the benefit of using an integrated configuration such as the INA592.
For less precise applications or where sufficient calibration is performed, a less precise op amp and higher
tolerance external resistors may fit the performance specifications required.
Disadvantages: Due to integrated resistors, the gain value is fixed for this integrated difference amplifier. In thecase of the INA592, the gain value is ½ or 2 (V/V). The current through the load can be varied by changing Rs;
however the fixed gains limit the range of values Vshunt can have for the same input voltage difference. This canresult in limitations in circuits with low supply voltages or large load resistors due to limited output headroom inthe design.

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Integrated Improved Howland Current Pump - INA592 and Settable Gain – Design 44 Integrated Improved Howland Current Pump - INA592 and Settable Gain – Design 4
VS+
VCC
IN-

12 k

6 k

INA592
SENSE

+
Vn
–

OUT

+
GND
IN+

12 k

6 k

REF

Rs

+
Vp

+

+

VEE
VS+
+

A

–

VS+

IfeedbackV
A

IshuntA

Iload

Vshunt

+

GND

+

VSRg
Rf

Rload
V_termination

Figure 4-1. Integrated Improved Howland Current Pump With INA592 and Settable Gain
Figure 4-1 shows the same integrated design of an Improved Howland current pump as Integrated Improved
Howland Current Pump - INA592 and Buffer – Design 3; the only difference being that the feedback op amp has
a settable gain compared to the unity gain of the buffer configuration. This gain configuration gives the ability
to design for different values of Vshunt for the same input voltage difference, which was limited with Integrated
Improved Howland Current Pump - INA592 and Buffer – Design 3. Iload can now be calculated using Equation 5.

(5)

(6)

Benefits: The benefits to this configuration are the same as in Integrated Improved Howland Current Pump INA592 and Buffer – Design 3; with the added benefit of being able to vary the Vshunt voltage as discussed
above. Having the ability to adjust the voltage across the shunt resistor allows the circuit designer more freedom
to set the output headroom of the circuit while still benefiting from the precision and high performance of the
INA592.
Disadvantages: One disadvantage with this configuration could be the size of the circuit since two op amps
and two external resistors are used in the design. This also leads to a slightly more complicated design thanthe previous three configurations. Due to non-ideal external resistors (Rf, Rg), gain error can be expected, which
affects the current accuracy of the circuit.

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Design Needs and Considerations

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5 Design Needs and Considerations
The need for a simple voltage-controlled current source can be readily met by implementing the Improved
Howland current pump topology and due to the versatility, there are many options to choose from as discussed
previously. When designing for a specific application there are many parameters to consider. With all four
configurations, non-ideal characteristics of any op amp and resistor are an inherent source of error in thedesign. One non-ideal characteristic of an op amp to consider is the offset voltage. The effect this non-ideal
characteristic has on the final performance of the Improved Howland current pump circuit can be significant.
Using precision op amps with very low offset voltage (< 100 µV) considerably reduces the error the circuit
contributes. The offset voltage information is included in the electrical characteristics table of the op amp data
sheet.
Another non-ideal characteristic of an op amp to consider is the output voltage swing limitations as the output
current changes. When considering output headroom performance, refer to the typical values given in the Output
Voltage Swing vs. Output Current graphs in the data sheet of the op amp. Doing so allows one to account forthe output swing limitations. Similar considerations must be taken to make sure the common-mode input voltage
range of the op amp is not violated. For designs 2 through 4, account for input and output swing limitations
for both op amps. These input and output limitations contribute to the overall voltage compliance of the current
source.
As mentioned throughout the article, each design has disadvantages and depending on the specific design
goals, one design can be more desirable for an application than another design. Considering parameters suchas the supply voltage of the amplifier, output impedance, thermal noise due to resistors, and the op amps, theamount of freedom to design for output headroom, and overall accuracy are a good start when narrowing down
which design to use.
Use Table 5-1 as a starting place when choosing which design and the level of precision of op amps to
implement. In some cases implementing a general purpose op amp, such as the OPA310, OPA2310, OPA990,
or OPA2990, is enough for specific design goals compared to more precise op amps like the OPA192 or
OPA2192.
Table 5-1. Comparison Table for Designs 1-4
DesignAmplifiers

Device Supply Voltage

OutputImpedance

ThermalNoise

Designing for
Headroom

Accuracy

1.A

OPA310 Op Amp

1.5 V to 5.5 V

R3+R4

Varies

Best

Good

1.B

OPA990 Op Amp

2.7 V to 40 V

R3+R4

Varies

Best

Good

1.C

OPA192 Op Amp

4.5 V to 36 V

R3+R4

VariesBest

Better

2.A

OPA2310 Op Amp

1.5 V to 5.5 V

HighModerate

Best

Better

2.B

OPA2990 Op Amp

2.7 V to 40 V

High

Moderate

Best

Better

2.C

OPA2192 Op Amp

4.5 V to 36 V

High

Low

Best

Better

3.A

INA592 with OPA990 Buffer

4.5 V to 36 V

High

Moderate

Good

Best

3.B

INA592 with OPA192 Buffer

4.5 V to 36 V

High

Low

Good

Best

4.A

INA592 with OPA990 Feedback Op
Amp

4.5 V to 36 V

High

Moderate

Best

Better

4.B

INA592 with OPA192 Feedback Op
Amp

4.5 V to 36 V

High

Low

Best

Best

When narrowing down which configuration to use, consider the error, cost, and size budgets of the system.
Additionally, consider design specifications such as the current required through the load, output headroom, andvoltage constraints for a specific design.

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Operational Amplifier Considerations

6 Operational Amplifier Considerations
Table 6-1 lists the operational amplifier considerations. Table 6-2 details the integrated difference amplifierconsiderations.
Table 6-1. Operational Amplifier Considerations
Part Number

Specifications

OPA310 High Output Current Op Amp (1.5 V to 5.5 V)

Rail-to-rail input and output (RRIO), ±150-mA output short circuitcurrent at 5.5 V, ±250-µV offset (±1.3-mV max), low 16 nV / √Hznoise at 1 kHz

OPA192 Precision Op Amp

RRIO, low ±5-µV offset (±25-µV max), low 5.5 nV / √Hz noise at 1
kHz

OPA191 Precision Low-Power Op Amp

RRIO, low ±5-µV offset (±25-µV max), low 15 nV / √Hz noise at 1
kHz

OPA197 Precision Op Amp (for cost-optimized designs)

RRIO, low ±25-µV offset (±100-µV max), low 5.5 nV / √Hz noise at 1
kHz

OPA196 Precision Low-Power Op Amp (for cost-optimized designs)

RRIO, low ±25-µV offset (±100-µV max), low 15 nV / √Hz noise at 1
kHz

OPA990 General Purpose Op Amp

RRIO, ±300-µV offset (±1.5-mV max), low 30 nV / √Hz noise at 1 kHz

Table 6-2. Integrated Difference Amplifier Considerations
Part Number

Specifications

INA592 Precision Op Amp

RRIO, low ±14-µV offset (±40-µV max), low 18 nV / √Hz noise at 1
kHz (G = ½)

INA597 Precision Op Amp (for cost optimized designs)

RRIO, low ±14-µV offset (±200-µV max), low 18 nV / √Hz noise at 1
kHz (G = ½)

INA1620 Op Amp with integrated precision resistors±100-µV offset (±1-mV max), Ultra-low 2.8 / √Hz noise at 1 kHz

7 References
•
•
•
•
•

Texas Instruments, AN-1515 A Comprehensive Study of the Howland Current Pump
Texas Instruments, Parallel Amplifiers for Higher Output Power: An Improved Howland Pump Approach
Texas Instruments, TI Precision Labs Noise Video: 8-1 TI Precision Labs - Op Amps: Noise - Spectral Density
Texas Instruments, TI Precision Labs Stability Video: 10.1 TI Precision Labs - Op Amps: Stability Introduction
Texas Instruments, Importance of Matching Difference Amplifier Resistors: Difference Amplifiers – the Needfor Well-Matched Resistors

8 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision * (October 2020) to Revision A (February 2023)
Page• Added references to OPA310 and OPA2310 in sections 5 and 6.......................................................................9• Added reference to Parallel Amplifiers for Higher Output Power: An Improved Howland Pump Approach ...... 9

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