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

#GUID-CF7FA493-5A4B-433B-B4C1-A8B6C87C6609 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. Load current can accurately be represented by GUID-78110063-CBBF-4E9E-B325-D88D0AAF8E38.html#GUID-573B0A03-C43B-458A-ADCA-DFD01C422A3D; 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 #GUID-3FE77835-F319-4EFB-B85C-22ECC63E0F0C is that it minimizes many of the sources of error seen in the discrete designs. The buffer creates high output impedance practically eliminating I_feedback 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 typical gain 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 high performance 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 the case 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 V_shunt can have for the same input voltage difference. This can result in limitations in circuits with low supply voltages or large load resistors due to limited output headroom in the design.