Multiplexing analog signals

Question

I am designing a system to measure voltages from bio-potential sources and feed them into an ADC after filtering and amplification. The source has the following charactristics:

1. amplitudes are in 10's of uV
2. frequency band of interest is 3Hz to 50Hz. 3.The 2 differential inputs from the bio source would have a DC offset (sometimes as high as 900mV)

As it happens, the vast majority of other designs filter, amplify and then feed the signal into a MUX, which in turn connects different signals to the ADC to be read, something like this if we had 2 sources:

^{simulate this circuit – Schematic created using CircuitLab}

The first thing that went through my mind, was that depending on the application, the op amps used in filtering stages will be expensive.

Is there something fundamentally wrong connecting the sources to the MUX and then feed them into the filtering circuitry? like this:

^{simulate this circuit}

Highest frerquency of interest is 50Hz, am i wrong in assumming that the response time (transient response) of even an oridnary filter would be more than sufficient for such a low frequency?

Answer 1

Analog Mulitplexors, such as MC4051 available from several fab houses, will do just fine. There are quad 2:1, dual 4:1, etc in 14/16 pin packages.

The random thermal noise in a 100 Hertz bandwidth, with 1,000 ohm channel resistance, will be 4nanoVolt * sqrt(100 H) = 40 nanoVolts.

Any 1/F noise may be higher, since these muxes are not designed to minimize that crystal_defect behavior.

By operating at +- 5 volts (that is, use a -5v on the VEE? and +5volt on the VDD?), your switch will easily handle 0.9 volts of DC.

You need to use a Ground plane, and have substantial R+C filtering on the several power rails. You also need R+C filtering on the Channel Select inputs, to exclude MCU trash ( 0.5 volt or so, from internal switching as the program continues to execute).

To protect against ESD, EMI, RFI, I would insert a low_pass_filter in each input from your sensors: 1Kohm resistor and 0.1uF capacitor (non-polarized ceramic, using the GROUND plane (you are using one, right) as the Return side of the energy_shunting 0.1uF.

For confidence, you might examine the instruction manual (repair manual) of commercial equipment, and learn how they protect the inputs.

Answer 2

There should not be anything wrong other than you may require different filters for the different sources. If this is not the case then doing the filtering at the end of the chain is not uncommon. Actually it may be even best done by digital filtering after your ADC. However if 50Hz is your highest frequency what is the lowest, DC ? It may however depend on other things such as your resolution required 8, 10 or 16 bit ? If you're considering purely low frequency audio and no DC, there are audio codecs with built-in digital filters that may be adjusted as required, even according to your selected source requirements ...

Answer 3

The main problem is that if your switching frequency is 500kHz, and the bandpass filter limits the signal to a maximum of 50Hz, then it will completely trash both signals. You will get a bandpass filtered version of what you get if you mix the two signals together.

The signal seen on the input of the bandpass filter will be rapidly switching back and forth between the two inputs. Imagine at some moment, for a few milliseconds, signal 2 is +10uV and signal 1 is -10uV. The bandpass filter will see a 500kHz signal of amplitude 20uV peak-to-peak. Since 500kHz is way outside its 50Hz range, it will remove that signal, and you'll get 0uV out.