SCEL: IPO8 isolated potentiometric output

The SCEL:IPO8 is a SPLat Circuit Element Library design for simulating a potentiometer connected to the control input of some external device. It provides an isolated output. A typical application would be a speed control input. This is a DC output circuit, and is not suitable for AC output applications such as isolated audio volume control. The circuit is suitable for inclusion on a custom SPLat controller or on a SPice board.

The SCEL:IPO8 has 3 connections to the outside world. 2 of these are for an externally supplied reference voltage (corresponding to the "top" and "bottom" of the simulated potentiometer) and the third is the "wiper" of the simulated potentiometer. Within the operating limits of the circuit, the "wiper" voltage will equal the DC reference voltage times the ratio command supplied by the SPLat program. The "ratio command" usually means an analog output instruction where the value put out is treated as a proper fraction (i.e. <1) of the full scale value.

The circuit was initially designed to simulate a control potentiometer on an Engine Control Module (ECM). It was driven off the 3.7kHz PWM output on an MMi99 SPice connector. In that case the reference voltage was 5VDC (supplied by the ECM) and the wiper voltage range of interest was 1V to 2V with a wiper load current of 6mA. The resultant circuit performs well with output voltages between 0V and 3 to 3.5V (depending on loading). It has good linearity over the active range, low output impedance and a low temperature sensitivity. Response time is in the 100mS range.

In the following characteristic data the "typical" figures are results of prototype testing. The "worst case" figures are our estimates of worst case performance, but are not guaranteed. When negotiating an implementation of this, or any other, circuit element in your custom SPLat controller, you must identify the parameters that are most critical.