The SCEL:DFRO1 is a SPLat Circuit Element Library design that provides a mid-rail referred ratiometric voltage output. It was designed to drive a voltage controlled hydraulic valve requiring a control voltage which varies between 25% and 75% of the supply voltage (e.g. Danfoss).

The circuit appears to the SPLat program as an analog output and a digital enable output. When disabled the output voltage goes to midrail. When enabled the output varies between approximately 20% of supply for a zero analog output to approximately 80% of supply for a full scale analog output. (Hence the output range is 5% either way in excess of the 25%-75% expected by the valve). The circuit works well with supply voltages from 10V to 28V. It is designed for a load with an equivalent circuit of 12kOhms to midrail.

In the following characteristic data the “typical” figures are results of prototype testing. For the tests the circuit was driven off the 3.7kHz PWM output on an  MMi99 SPice connector. (8-bit resolution). 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.

One point to keep in mind is that there is no analog output value that corresponds to exactly ½ scale. There are exactly as many values below ½ scale as above, but no value at ½ scale. For example, in an 8-bit system, there are 128 values below ½ scale and 128 above. The mid-scale value would be 127.5. For this reason, the mid-scale offset figures quoted are the value that would be obtained if the analog output was 127.5. That said, the circuit could be supplied with a trimpot for setting a precise mid-point value.