Custom Test Circuit Interface


Custom test circuit interface
What is the challenge?

The modern car is full of sensors that continually monitor the operation of the vehicle. The onboard computer reads these signals, thousands of times every second, and makes adjustments to ensure the vehicle is operating efficiently and reliably. Some of the sensors use digital logic signals in the range of 5-12 volts. Other sensors use Variable Reluctance signals capable of outputs exceeding 100 volts. As technology improves, these voltages are expected to increase. We decided on +/- 350 volts as our maximum input range. The sensors themselves, must be tested to make sure they are giving accurate information to the computer. The output voltage is compared against the sensor specification to verify the signal is correct.

Recently, we have seen sensors with multiple outputs come onto the market. We wanted to be able to test multiple outputs simultaneously, in order to keep up with future sensors.

Above: The entire test system, showing the custom PCB, the dedicated

The challenge is there were no commercially available power supply, the analog input units, and the control board to control

measurement systems capable of this voltage range. the routing relays.

In fact, most commercial solutions were limited to

voltages of +/-10 volts or less. Additionally, in order to accurately test the timing of sensors, we needed a circuit that was at least 10 times faster than the fastest sensor. These requirements resulted in the development of a custom test circuit.

Custom test circuit interface

What are the project requirements?

1. Accurately capture signals as high as +/- 350 V.​​

2. Maintain a 50 MHz bandwidth for high speed signals.

3. Capture low level signals of 0 to 5 V with good accuracy.

4. Maintain a low noise floor.

5. Capture 4 channels simultaneously.

​​The easiest solution would have been to simply reduce the high voltage signal with a voltage divider. However, the voltage division resistors would have made the signal unable to drive the input of the ADC while maintaining the 50 MHz bandwidth requirement. As well, reducing the Above: 200 mV signal, after 100:1 reduction.

0 to 5 V signal by the same amount would have made that Vertical axis = 5 mV/Division

signal too small. (It would have been picked up, but there

would be an unacceptable amount of noise.)

Below: 20mV signal, after 100:1 reduction. How did Genesis Automation meet the

Vertical axis = 200 µV/Division Challenge?

Amplitude vs. Samples chart

​​1. Reduce the signal’s voltage from as

high as +/- 350 V down to the

converter’s input range of +/- 10 V.

Solution: Attach a 100:1 voltage divider to

bring the output voltage down to +/- 3.5 V.

2. Maintain the 50 MHz high speed signal.

Solution Step 1: Attach a high speed

operational amplifier (op amp), with a

rated bandwidth of 130 MHz, to

buffer the output of the 100:1 voltage

divider.

Solution Step 2: To compensate for the

input capacitance, place a variable capacitor across the voltage divider.

3. Capture low level signals of +/- 2 V with good accuracy.

Solution Step 1: To handle the low level signal, Genesis Automation engineers built an identical circuit with a

4:1 voltage divider as well as a straight pass-through option on the board.

Solution Step 2: Attach a network of switching relays on the board to direct the signal as needed.

4. Maintain a low noise floor.

Solution Step 1: Use high quality low noise resistors to divide the signal.

Solution Step 2: Use low noise op-amps to buffer the signal after it is reduced.

Solution Step 3: Use careful circuit layout to avoid adding noise to the signal.

During testing, Genesis Automation engineers were able to recover a 20 mV signal, sent through the 100:1

reduction circuit. This represents a signal to noise ratio of 94dB. The ADC itself is only capable of 96dB, so the

circuit added less than 3dB of noise at all ranges.

What are the origins of this technology?

The whole concept behind a voltage divider circuit comes from the input of a standard oscilloscope. The Genesis team used that circuit and modified it for their purpose.

What makes this project unique?

The ability to accurately capture high voltage, high frequency signals is something that could only be done with custom purpose built circuitry.

Below: Circuit Design 3D render. The white boxes represent the Below: The final production PCB, as installed in the system. variable capacitors that would be mounted on the back side. The 4 inputs are on the top. The 6 outputs are across the The black rectangles are 26 routing relays. bottom. 2 of the outputs are BNC for high speed connection to the 50 MHz inputs, while 4 of the outputs are screw terminal for precision 2 MHz inputs. The gray cable is used to control the

routing relays.

Custom test circuit interface
Custom test circuit interface

When your project requires intricate design and engineering, contact Genesis Automation.

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