Attenuate Automotive Voltage Levels

When a voltage exceeds your equipment’s limits, you can build a resistive voltage divider to reduce the signal level. The circuit in Figure 1a sets Vout = Vin * R2/(R1 + R2) and works well for single-ended measurements. The circuit in Figure 1b sets Vout = Vin * R2/(R1+R2+R3) and works if you need differential measurements.

Because voltage dividers introduce errors to your signals, I recommend using metal-film resistors with 1% tolerances. You should also calibrate your dividers by measuring Vin and Vout. Use that ratio in your software to scale the measured voltage back to its original value.

Some Assembly Required
You can assemble your voltage divider on a circuit board, but I suggest you build it into a small tube with connectors at both ends (Fig. 2). The tube lets you place the voltage divider in-line with your signal cables. Write the measured input-output ratio on the outside of the tube.

Be aware that the resistive voltage divider will produce loading-effect and settling-time errors. For example, a 1/3 voltage divider consisting of a 10-k resistor and a 30-k resistor will place a 40-k load on your signal source. Such a low impedance could cause measurement errors on automotive signals such as sensor outputs.

Simply increasing the resistor values to decrease the divider’s loading effect won’t always work. A divider consisting of 330-k and 990-k resistors, instead of 10-k and 30-k ones, will decrease the load on a signal source, but it may increase your instrument’s settling time, which will cause measurement errors as your data-acquisition system scans among channels.

To demonstrate the settling time problem, I programmed a National Instruments DAQPad-1200 to measure 14 V on one test channel, and I connected another channel to analog ground. Then, I ran the system with both a 330-k/990-k voltage divider and a 10-k/30-k divider. The 330-k/990-k had a maximum error of 785 mV when it performed a two-channel scan, and the error reduced to 1 mV with the 10-k/30-k divider.   

The 330-k/990-k divider’s resistors combine with capacitance on the data-acquisition system to create a long time constant that prevents the data-acquisition input from settling during the scan period. National Instruments recommends that you keep the output impedance that the DAQPad-1200 sees at or below 1-k for multichannel scanning, so even my low-impedance divider exceeded the manufacturer’s recommendations.

Get the Load Down
You can eliminate the voltage divider’s loading effects if you add an amplifier front end to the divider. The circuit in Figure 3 makes loading error insignificant because the amplifier presents a high-input impedance— typically hundreds of megohms—to your signal source. And the amplifier’s low output impedance (under 100 V) lets you select low-value divider resistors that eliminate the settling-time problem. The amplifier also lets you use the system’s single-ended configuration because the amplifier can perform the differential measurement, which doubles the number of input channels per units.

Figure 3. An instrumentation amplifier provides a high input impedance and a low output impedance, which eliminates loading effects caused by the divider resistors. We use the Burr-Brown INA103 amplifier, which sells for around $10.

Unfortunately the amplifier requires a power source and a circuit board; you can’t use the tube installation like you can with the resistive divider. Your power supply should be about 2 V over the largest signal you want to measure. I recommend that you purchase a ±24-V linear power supply. If you don’t have AC power available for the supply, you can use a PCB-mountable mini-switching supply that converts 5 V to the voltage you need for the amplifier. You can often use the data-acquisition system to supply +5 V for the switcher.

Although an amplifier can eliminate the problems caused by the resistors, poor grounds on a vehicle can cause measurement errors. Use an isolation amplifier, which costs about $150 per channel, when you have unskilled people connecting signals to the data-acquisition system. Isolation amplifiers also let you operate the data-acquisition system in single-ended mode.

In addition to the obvious disadvantage of cost, isolation amplifiers are available in fixed ratios of voltage division only. Custom units are available, but they may not be cost effective in small quantities. If you use an isolation amplifier, I recommend one that converts a ±20-V input to a ±5-V output.

Isolation amplifiers also limit the bandwidth of your measurements. At 10 kHz, for example, a wide-bandwidth isolation amplifier will reduce the input signal by 3 dB to 70.7% of its original amplitude. Signals with harmonics that exceed 10 kHz will experience greater losses. T&MW

Dave Robins is the president of Intrepid Control Systems; drobins@intrepidcs.com.