Application Circuits
8
SBOU175 – October 2019
Copyright © 2019, Texas Instruments Incorporated
OPA462EVM
The same circuit shown in
is applied, except the 2.5-VDC VP voltage source in
is
replaced by an 100-Hz ac source having a peak voltage of 2.5 V. The peak output current depends on the
peak voltage level applied at the VP input. A triangle wave is used in the example; however, a sine wave,
square wave, or other waveform could be applied instead.
shows that a 2.5-V peak input triangle
wave applied to the current pump input, VP, results in a 20-mA peak triangle wave at the output, Vout.
Figure 3. Improved Howland Current Pump Applied as a Peak AC-Current Generator
The improved Howland current pump op amp circuit relies on both negative and positive feedback for
operation; more negative feedback than positive feedback, but that feedback does not always provide
stability. When unity-gain-stable op amps, such as the OPA462, are employed and they drive a resistive
load, the op amp phase margin should be sufficient such that the circuit is stable. However, if the output
load is complex, containing both resistive and reactive components (R±jX), there can be combinations that
degrade the phase margin, and instability results. Degradation of the phase margin often occurs when the
current pump is used to drive loads that are predominantly inductive.
The OPA462EVM has a number of positions where component may be added to stabilize the various
OPA462 amplifier and improved Howland current pump configurations that are supported by the EVM.
Some of the component locations that are used for this purpose are R6 (ISO), R5 (Rng), C12 (Cng, noise
gain), R10 (Rsn), C16 (Csn, snubber), and C9 and C10 in the feedback loop. Compensation, when
required, is determined based on the particular circuit to which the OPA462 is being applied. Op amp
stability and compensation is a vast subject covered in numerous TI documents, and TI training programs
such as
.
