Signal conditioning basics

[pallisi]

Hallo everybody,

I am involved with a project that requires the measurement of a current as an input to a mcu. But I am a little confused about the signal conditioning techniques. There are people tha use instrumentation amplifiers, buffers, diodes etc. I would like to read some stuff about signal conditioning and generally how to connect analog inputs to mcu for best accuraccy and low noise.

Thank you in advance!

[T3sl4co1l]

First thing you need is something to convert the current to a voltage.  That is... a resistor (V = I*R).  If the current sense resistor has a superimposed voltage (a "high side sense"), you will need a differential amplifier as well, such as a current sense amp (there are purpose-made ICs for this), or an instrumentation amplifier.

Some sort of EMC protection and filtering, and acquisition bandlimiting, is almost certainly required.  Normally, you'd have a series resistor between the current-sensing shunt and the circuit.  Then some clamp diodes (e.g. BAV99) to the supply to contain ESD and spikes.  Then some capacitance (and subsequent RC/LC or active op-amp filtering) to limit bandwidth, further contain outside surges, and make things nice and smooth for the ADC readings.

The cutoff frequency of all that filtering is generally on the order of the sample rate.  It doesn't have to be below half (i.e., the Nyquist frequency).  There are very good reasons why you might want it above the sample rate.  Control loops (minimal latency, frequency-domain aliasing not a concern) are one, equivalent-time sampling (intentionally using aliasing to reconstruct a faster signal) is another.  If you do want to reconstruct the bandlimited signal (i.e., satisfying the Nyquist Sampling Theorem), you need a filter that excludes everything* above Fs/2.

*How much depends on accuracy; 40dB attenuation is 1% error (due to possible aliasing), and etc.  You can use a crappy filter, pretty far from Fs/2, to achieve this; or a pretty sharp filter, relatively close to Fs/2 (usually Fs/3 to Fs/4, with a 3-7 order filter -- depending on filter type), to maximize bandwidth or minimize step response (while retaining "real time" (i.e. no aliased or equivalent-time-sampling phenomena) response).

Tim

[bugs]

It all depends on your requirements and where in your circuit you want to measure. Do you have any requirements or schematics?
For example: LEM sensors are quite easy to use, have a reasonable bandwidth and isolation, but they can suffer from drift and may be influenced my outside magnetic fields. If you can do the measurement in the ground path, unisolated, a current shunt might be better for you, but you must have a good kelvin measurement. Etc.

[pallisi]

Thank you for your very useful information. My purpose is to design a measuring system for an inverter in order to implement open/closed loop control of an induction motor and overcurrent protection. The inverter will provide 540V/ 10 A to the motor.

I have reviewed quit a lot of configuration about current measurements. I am in a dilemma about using hall effect or a shunt resistor. I know the pros and cons about the two configurations. Of course the isolation as well as the size of the transducers are of great importance. I was thinking of using allegro current sensor (hall effect) or isolated current sensors from avago.

http://www.avagotech.com/pages/en/optocouplers_plastic/plastic_miniature_isolation_amplifier/acpl-790b-000e/

My main concern is the connection of the current sensing device to the mcu. I would like a reliable connection with minimum of noise and a maximum of protection. Any suggestions or experience would be useful.

[Zero999]

If the current sensor is isolated then there should be no need for an isolation amplifier.

What about a current transformer? Although 10A is a little low, you can use more turns on the primary.