Hi all,
I am building a circuit to provide temperature measurements from a number (16 to be precise) sensors. The idea I have is to use an analog MUX to switch between the 16 sensors, and an MCU to control which sensor is being read. In fact, I am using 3 MUX's - 2 x single 8 channel, and 1 x triple 2 channel MUX, the latter switches between the two 8 channel MUX's each of which controls 8 of the 16 sensors.
Anyhow, enough about the overall design - the question is more basic and concerns the behavior of the 8 channel MUX.
What I was expecting was that the voltage applied at the 'input' pin would be passed through unchanged to the 'output' pin of the MUX, providing that the relevant control signals were present, and the 'input' pin was correctly selected. Instead, I am seeing a different voltage presented on the 'output'. I have put together a simpler circuit which demonstrates this, and is representative of the overall circuit. A hand-drawn sketch is provided.
Referring to the sketch, the salient aspects are as follows:
- power is supplied from an Arduino and is 5 volts
- a CMOS MUX (CD4051) is used to switch between the 8 channels...but in the test circuit, I have only connected channel 0.
- there is a simple voltage divider based on a 120Ω resistor and a thermister - the latter varies around 120Ω at room temperature and the output is connected to the MUX channel 0
- the controls (A,B,C) are all set LOW which should select channel 0
- I measure the voltage 'inside' the voltage divider, which changes according to temperature
Now, if I earth the circuit at point A (which is channel 0 on the MUX) I get a reading of 2.42V from the voltage divider. This is what I would expect, around half of the 5V. However, if I then move the earth to point B, which is the output channel of the MUX, then I get a much higher reading of 4.22V.
I thought that the MUX was meant to have almost zero resistance when measured across the connected channels, so why does the MUX appear to be adding a significant resistance to the circuit?
I am building a circuit to provide temperature measurements from a number (16 to be precise) sensors. The idea I have is to use an analog MUX to switch between the 16 sensors, and an MCU to control which sensor is being read. In fact, I am using 3 MUX's - 2 x single 8 channel, and 1 x triple 2 channel MUX, the latter switches between the two 8 channel MUX's each of which controls 8 of the 16 sensors.
Anyhow, enough about the overall design - the question is more basic and concerns the behavior of the 8 channel MUX.
What I was expecting was that the voltage applied at the 'input' pin would be passed through unchanged to the 'output' pin of the MUX, providing that the relevant control signals were present, and the 'input' pin was correctly selected. Instead, I am seeing a different voltage presented on the 'output'. I have put together a simpler circuit which demonstrates this, and is representative of the overall circuit. A hand-drawn sketch is provided.
Referring to the sketch, the salient aspects are as follows:
- power is supplied from an Arduino and is 5 volts
- a CMOS MUX (CD4051) is used to switch between the 8 channels...but in the test circuit, I have only connected channel 0.
- there is a simple voltage divider based on a 120Ω resistor and a thermister - the latter varies around 120Ω at room temperature and the output is connected to the MUX channel 0
- the controls (A,B,C) are all set LOW which should select channel 0
- I measure the voltage 'inside' the voltage divider, which changes according to temperature
Now, if I earth the circuit at point A (which is channel 0 on the MUX) I get a reading of 2.42V from the voltage divider. This is what I would expect, around half of the 5V. However, if I then move the earth to point B, which is the output channel of the MUX, then I get a much higher reading of 4.22V.
I thought that the MUX was meant to have almost zero resistance when measured across the connected channels, so why does the MUX appear to be adding a significant resistance to the circuit?
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