Resistance-to-ground sensing with adaptive current source

[martinroger]

General operating concept

Most of the usual resistive sensors used on cars are implicitly connected to the general chassis ground, meaning that they are systematically and irrevocably placed on the low side.

The most accurate and stable way to measure the resistance of such sensor is to feed it a known - small - current and measure the resulting dV with an ADC behind a protection diode.

Edge cases and protection

Aside from the usual ESD protection, the current feed line for the resistive sensor must be able to endure the common misuses and abuses :

• Shorting to ground : current must be limited to prevent burning the current source (and ideally this should be detectable)
• Reverse voltage application : current must remain limited even if somehow a negative (relative to ground) voltage potential is applied instead of the resistive load.
• Overvoltage : the current source and sensing sources must remain functionally protected in case a higher-than-expected voltage is applied for a significant amount of time at the sensor feed point ( i.e. for longer than the pulses that can be cause by an ESD protection circuit )
• Open circuit - large resistance : in case of too large resistance value, or open circuit at the resistive sensor, the current source is lacking a connection to the sink. Means of detection must be set up to prevent this from lasting too long, and the circuit should be switched off in those cases.

Application-specific requirements

This project typically needs to measure two ranges of resistive sensors :

• Lower impedance sensors such as the fuel level sender, with typical resistances between 0 and 270Ohm
• Higher impedance sensors, such as some temperature sensors, with typical resistances between 0 and 2.7kOhm, sometimes extending beyond 5kOhm (with minimal value in being able to measure this far).
• For a measurement point there should be a way, preferably over I²C, to select which type of impedance range to measure.

Op-Amp-based implementation with switchable caliber

This implementation uses a voltage reference Vdd-Vref (immune to supply level variations) to drive an Operational Amplifier U1 that has a current-setting circuit on the high-side of a P-MOSFET M1.
The MOSFET low side is protected by a low drop diode D1, and the resistive load is connected between the diode and ground at Vsensor.
The ADC pick-up point is set before the protection diode at Vadc, creating a small voltage offset that is mostly stable with current and less so with temperature. This offset is compensatable via software.
This implementation is best with a rail-to-rail op-amp for simplicity like AD8542, and a high ADC input impedance to avoid leakage of measuring current in a voltage divider at the ADC input.
An additional MOSFET M3 with high gate impedance R11 can be used to add a second setting resistance RsetLowR in parallel to the original one RsetHighR in order to increase the source current for lower impedance sensors.

In the pictured set-up, in low impedance mode (M3 gate to ground) the source current is about 9.01mA all the way to sensor resistances nearly up to 310 Ohm.
In high impedance mode (M3 gate to Vdd) the source current is about 1.08mA all the way to sensor resistances nearly up to 2.8 kOhm.

Constant current source IC implementations

LED drivers

Most I²C-settable LED drivers that can be reduced to about 1mA per channel are usually of the current-sink or open-drain layout, which prevents placing the resistive sensor on a driver channel, as the current sensing is done on the low side (as opposed to on the high side as in the Op-Amp-based implementation.

Some implementations with I²C and high-side current-setting unfortunately are rated for much higher voltages or very large currents (typically 1A) and low precision, thus not very useful for the task at hand.

Others ?