Isolation amplifiers are used as analog interfaces between systems with separated grounds. This tech report includes the design of a novel dual-channel isolation amplifier with high linearity and low drift, with a design program in MathCAD. Included topics are detailed design equations, linear-optocoupler nonlinearity, optocoupler amplifier analysis, and closed-loop transfer functions for computing dynamic response.

Test data is also included for reference and applies to prototype hardware. These iso-amp designs can be used to derive other iso-amp designs for specific requirements.

Tech Report Contents

	Isolation Amplifier Schematic Diagram, in HTML
	Photodiode transresistance amplifier program, in HTML and MathCAD
	Isolation Amplifier theory: detailed circuit explanation, test data, derived design equations for dynamic response
	Linear optocoupler data sheets

Content Overview

Large-Signal Linearity

One of the main performance obstacles of isolation amplifiers is large-signal nonlinearity or distortion. The high-performance isolation amplifier applies linear optocouplers (LOCs) differentially to increase linearity over a large signal range. An LOC is like a generic optocoupler except that it has two matched, monolithic photodiodes, one for feedback to the LED driver and the other for the isolated output. LOCs are supplied by TI, Agilent, CP Clare, and Infineon (Siemens). The iso-amp uses a novel dual-feedback circuit topology to significantly reduce distortion.

The above distortion analyzer residual output was displayed on an oscilloscope for a single-LOC isoamp with a 4.8 V pk-pk, 100 kHz input, offset to -4 V. The prevalent second harmonic is caused by the LOC nonlinearity, which is an even function. The total harmonic distortion (THD) is 2.8 %, considerably worse than 0.16 % THD for the differential iso-amp. 

At the other end of the range, for +4 V offset, single-LOC THD was 2.45 % while for the differential-LOC was 0.15 %. The approximately 20-times improvement at 100 kHz demonstrates that for limited loop gain, open-loop nonlinearity compensation significantly reduces distortion.

Because the LOCs are at the output of the feedback amplifier, consisting of forward  and feedback path op-amps, linearity error is reduced by the loop gain. Because of op-amp single-pole roll-off, loop gain decreases with frequency, and with it, linearity. Consequently, linearity is maximized at higher frequencies by making the open-loop response as linear as possible. This is accomplished in this design by compensating the inherent nonlinearity of the LOCs.

The LOC gain curves are convex, as shown in manufacturer's current-transfer-ratio plots.

As LOC LED diode current increases, gain increases sublinearly, as shown above. At lower drive currents, deviation from a linear gain is nearly equal to the deviation at higher currents. Feeding back and summing both LOC outputs tends to cancel these errors, linearizing the loop. 

LOC photodiode matching recreates the feedback photocurrents at the outputs. Photodiode op-amps amplify the differential output and drive a one-op-amp diff-amp. Optionally, for dc amplification, a biFET op-amp ´1-inverts for an in-phase output while the output op-amp stage provides independent gain and offset adjustment, and a 50 W output.

One of the challenges in isolation amplifier design is to minimize drift. As temperature rises, for constant LED current, photocurrents decrease. Ambient temperature affects both differentially-configured LOCs, causing their gain curves to tend to track. Feedback and output (mutual) photodiode currents of each LOC are matched, and temperature effects track on the output side with the feedback loops on the input side. As an LOC heats, its photocurrent decreases, causing its feedback voltage to change in a direction that servos the LED-driver op-amp output to produce more photocurrent to maintain a nulled error at the input. Consequently, feedback corrects temperature drift for each LOC separately. The output voltage TC (DV/DT) due to heating of LOCs in both the single  +LOC and –LOC configurations was no worse than for the differential-LOC case.

For differential heating of the LOCs, the feedback amplifier responds to the drift by driving both LOCs to achieve error nulling at the input. Compensation is thereby achieved in part by changing the drive to both LOCs to compensate for their temperature difference.

An estimated 40 °C rise results in an output offset of less than ± 20 mV with the input grounded, or roughly a TC of less than 1 mV/°C. Gain TC was comparable for single-LOC configurations of both polarities.

For a 1 V pk-pk, 1 kHz sine-wave, a temperature rise of about 40 °C above ambient results in an increase in gain of about 1 %, or a gain TC of about 0.025 %/°C (250 ppm/°C).

Frequency Response

The iso-amp design is dynamically compensated for a maximally-flat amplitude response, with negligible step overshoot. The bandwidth is typically around 150 kHz. Time and frequency-response characteristics may be traded off by changing values of compensation capacitors on either the input or output sides of the amplifier.

The iso-amp tech report also contains the detailed dynamic analysis equations and MathCAD simulation files to aid in modifying the response. Faster op-amps can be used to extend bandwidth using the design tools supplied.

	high-speed: ³ 100 kHz; 150 kHz -3 dB bandwidth typical
	high-linearity: 0.02 % THD at 1 kHz; 0.1 % at 30 kHz, 1 V pk-pk out
	large dynamic range with low distortion; < 0.5 % THD for 5 V pk-pk input
	250 ppm/°C drift typical
	nominal unity gain; independent output gain (scale) and offset adjustments; 50 W output
	300 V isolation minimum, limited only by optocoupler ratings
	low manufacturing cost: < $10 parts cost
	uses differentially-driven linear optocouplers (LOCs)
	unique dual-path feedback topology reduces THD caused by LOCs by ´ 20
	feedback paths from differential LOCs cause error cancellation

Applications

	floating pulse amplifier output voltage and current interface
	instrumentation in high-noise environments
	isolation for audio and transducer signals
	high-voltage signal translation
	remote and test-cell signal acquisition
	analog front-end processing
	motor-drive and power converter current sensing
	medical instrumentation leakage barrier
	power-line-connected circuit measurements
	desktop computer interfaces

Iso-amp Ordering

Isolation amplifier tech report:

Ordering Information