A current-output monitor that clips before its full scale
Symptom
Section titled “Symptom”A current-sense monitor with a current output reads correctly at small currents, but above some current its output stops rising, below the full scale the design computed from its gain. A larger output resistor makes the reading clip at an even lower current.
The monitor forces the sense voltage across its input resistor and sends the resulting current through its output resistor, so its gain is the ratio of the two (current-output monitor) [Analog Devices, LTC6101/LTC6101HV]. Its output can rise only to a largest voltage; the LTC6101’s data sheet gives it as absolute values at stated supply voltages [Analog Devices, LTC6101/LTC6101HV]. The output resistor sets the gain and so the current at which the output reaches that voltage, but not the voltage itself. The full scale is the lower of that limit and the ADC’s full scale.
In the sense chain’s example, the monitor can drive its output to 4 V, and the ADC reads up to 2.5 V. At the largest current, 2 A, the output is 2 V, and 2.11 V with the pads and the offset at the sign that raises it. Doubling the output resistor doubles that highest output, past both limits.
How to confirm
Section titled “How to confirm”Raise the current in steps and read the output at the monitor’s own pin. A line that bends flat at a voltage that does not move when the output resistor changes is the output’s limit; compare it with the data sheet’s largest output at your supply voltage.
Choose the output resistor so that the output at the largest current, with the offset’s and the pads’ contribution at the sign that raises it, stays below both the monitor’s largest output and the ADC’s full scale. The sense chain tool computes that full-scale current and flags a reading that can clip.
References
Section titled “References”- [Analog Devices, LTC6101/LTC6101HV]: the current-output monitor, its gain from the two resistors, and its largest output voltage.
- Sense chain: the full scale and the output’s limits, worked through.
- Current sensing: the shunt and its connections.
The numbers in the examples, presets and exercises on this site are synthetic: round teaching values, not data from any real design.