Current Sensing

Need current sensing. This should be isolated because the maximum common mode voltage of most non-isolated parts might be around 100V, whereas current sensing of the DC bus could be ±450V,  and the peak AC voltage could be 358V (ignoring possible harmonics and spikes). There are three isolated options:

  • Shunt (series) resistor with the voltage measured with one of the following:
    • Sigma-delta modulator output to on-chip sigma-delta demodulator. This is not sampled and thus harder to synchronise. Two options for isolation, both of which require an isolated and non-isolated supply:
      • Isolated S-D modulator, connected directly to the microcontroller. TI AMC1204, AMC1304, AMC1305ACPL-C796J, .
      • Non-isolated S-D modulator, connected to the microcontroller through a digital isolator (optocoupler).
    • External ADC interfacing with the microcontroller with SPI. The digital SPI lines are passed through a digital isolator. There are external ADCs which are built for the small current shunt voltages (current shunt digital output). Alternatively, standard external ADCs will typically have an input range of around 5V, there is a need to have a circuit to amplify the voltage drop across the shunt resistor as this is typically in the order of mV. No amplification would lead to a substantial decrease in resolution of 4-5 bits (250mV of 1/20th of the ADC range). Amplification options include:
    • Isolation amplifier output to on-chip ADC. AMC1100, AMC1200 or ACPL-C790. The AMC1100/AMC1200 are made for current shunt measurements, so they have an input voltage range of ±250 mV.  Alternatively, there are similar products from Avago (read this document regardless if going down the isolation amplifier path as the FAQ section gives a good comparison with Hall effect sensors and talks about input anti-aliasing RC filters). They consists of a delta-sigma modulator input stage including an internal reference and clock generator. The output of the modulator and clock signal are differentially transmitted over the integrated capacitive isolation barrier that separates the high- and low-voltage domains. The received bitstream and clock signals are synchronized and processed by a third-order analog filter with a nominal gain of 8 on the low-side and presented as a differential output of the device. ISO121/ISO122/ISO124 are precision isolation amplifiers that use duty cycle modulation-demodulation. They have an input voltage range of around ±10V to ±12.5V so there is a need to have a circuit to amplify the voltage drop across the shunt resistor as this is typically in the order of mV. Amplification options include:
    • Analog linear optocoupler (1 LED + 2 photodiodes) with external op-amp circuitry. Two optocouplers are required for bipolar operation. Examples include HCNR200/HCNR201, IL300, LOC110. These analog linear optocouplers need to be combined with op-amp circuits at both the input and output of the optocoupler. These circuits need to shift and amplify the signals appropriately. This method is not suggested due to the complexity compared with other options.
  • Hall effect to on-chip ADC (preferred): There are a variety of different Hall effect current sensors. LEM LTS/LTSR/LTSP and CAS/CASR/CKSR are powered from a unipolar 5V supply (more readily available than a bipolar 15V supply). The output has a 0V to 5V range which is easier to interface with the on-chip ADCs than a bipolar output. As the input range of the ADC is 0V to 3.3V  the LEM output must be adjusted. This can be achieved in three ways:
    • Voltage divider on LEM output (with optional op-amp unity buffer). This maintains the full range of the current sensor, and just scales it down
    • Op-amp circuit to shift and scale (preferred). This allows a configurable gain with appropriate resistor value selection, which allows the ADC to use more of its range for improved resolution (see here). This also allows easy filtering of the LEM output and ADC input. This also accounts for drift in the LEM reference. For example, if the LEM reference drifts to 2.54V, the 0A output will still be 1.65V, whereas for the voltage divider this will become 1.6764V, introducing a DC offset to the measured current which may affect control.
    • Drive the reference of the LEM (on LTSR/CASR/CKSR models), to 1.65V which will make the output 0V to 3.3V. This method reduces the range of the current sensor (see datasheet)

Could I use multiple current sensors to give accuracy at different ranges? E.g. A hall effect sensor looped 5 times for

A single 6-8 input ADC could be used on the sensing board for simultaneous sampling of all currents and voltages, which then interfaces digitally with the MCU. The inputs to the ADC could come from LEM circuits or isolated amplifiers.

Signal conditioning means manipulating an analog signal in such a way that it meets the requirements of the next stage for further processing (e.g. scaling input voltage to input of ADC, converting between differential and single-ended signals).

Need for anti-aliasing filters!

To use a bipolar +/-12 V ADC (since it’s more suited for voltage sensing as shown in the next section, and the ADC comes with 8 input channels), we could use either:

  • Shunt resistor, filter, AMC1100 isolated amplifier (+/-250 mV in, 2.55 ± 2V differential output), followed by op-amp circuit to subtract 2.55V (or differential input) and then amplify by 8x (35A, 0.005 ohm resistor = 175mV, 8x gain = 1.4V, 8x gain = 11.2V). (costs £6.50 for resistor and AMC1100).
  • CKSR 15-NP Fluxgate sensor (2.5 ± 1.5V differential output), followed by op-amp circuit to subtract 2.5V (or differential input) then amplify by 8x (35A = 1.46V out, 8x gain = 11.67V) (costs £11.50 for sensor alone)
  • LA 25-NP or LAH 25-NP Hall effect sensor (± 35 mA output, with maximum 263 ohm burden measuring resistor = 9.2V max), then amplify by 1.5x. (costs £14-16 for sensor alone).