Help needed with CD4052 multiplexer input buffers. Something's wrong here...

First of all, hello! I've been a long time lurker here but am just beginning to post now.

I am designing a circuit in which a CD4052 analog multiplexer is controlled with a PIC microcontroller.

Any number of 4 of the CD4052's inputs can be connected at any given time. The PIC controls the multiplexer and inputs the multiplexer's output to an ADC channel.

For this reason, Schottky diode protection is necessary on the ADC input (and thus the CD4052's output)

Anyhow, my circuit is behaving pretty poorly and not producing the right voltage results on the multiplexer output.

Here is the test schematic I am using to try and debug this issue, please let me know if anything you spot is wrong! This is exactly how it exists on my breadboard at the moment.

The CD4052's A/B inputs are selecting x0 as the current output at pin13 (X output) -- there is a voltage there, however it is wrong.

Thanks in advance for any assistance. It is most appreciated as I have nobody locally that can take a look...

Anyhow, my circuit is behaving pretty poorly and not producing the right voltage results on the multiplexer output.

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Can you be more specific?
The TL074 is not a rail-to-rail opamp. Could that be affecting your results?

alec_t said:

Can you be more specific?
The TL074 is not a rail-to-rail opamp. Could that be affecting your results?

Click to expand...

Hi Alec,

Thanks for the response!

Sure can...to test whether it was just out of the voltage range for the TL074 I input a voltage of 3.254V DC to the circuit. I then read a slightly dropped voltage of 3.184 at the opamp's input and output. I then read a voltage of 3.169 at the CD4052 output, so there is a rather large voltage drop. At higher voltages, the magnitude of the drop is of course larger.

I hope this makes it clearer! Thanks for any input

Your 4052 error is ~ 15 parts per 3000, i.e. 0.5%. That's pretty good in my book .
What is the input resistance of your meter? The 4052 has a finite resistance which will act as a potential divider with the meter resistance.

alec_t said:

Your 4052 error is ~ 15 parts per 3000, i.e. 0.5%. That's pretty good in my book .
What is the input resistance of your meter? The 4052 has a finite resistance which will act as a potential divider with the meter resistance.

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Okay, I suppose I did not consider that the multiplexer had an error factor. I quite seriously thought that maybe it's just a digitally controlled switch...

.5% is definitely acceptable, but isn't the potentiometer to opamp drop out of spec? Surely something is wrong here, no?

Not sure on my meter's resistance. I've never actually measured it...how would I go about doing that?

isn't the potentiometer to opamp drop out of spec?

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Can't see a potentiometer in your circuit
Another source of voltage drop is R1. Have you taken that into account?

Not sure on my meter's resistance. I've never actually measured it...how would I go about doing that?

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1) measure the voltage of a known good alkaline cell. 2) measure it again but this time with a 1 megOhm resistor in series with one meter lead. 3) do a subtraction of the two measurements to calculate the voltage dropped by the 1meg. 4) the meter resistance is N megOhms if the second meter reading is N times the voltage across the resistor.

alec_t said:

Can't see a potentiometer in your circuit
Another source of voltage drop is R1. Have you taken that into account?
1) measure the voltage of a known good alkaline cell. 2) measure it again but this time with a 1 megOhm resistor in series with one meter lead. 3) do a subtraction of the two measurements to calculate the voltage dropped by the 1meg. 4) the meter resistance is N megOhms if the second meter reading is N times the voltage across the resistor.

Click to expand...

Sorry, my bad...I have a potentiometer I am using as a voltage divider to generate a test voltage with. That is where the 3.254V test supply is coming from.

So, I guess the main problem I have here is that I am getting a completely different voltage than the source voltage on the output. It is important for a user to be able to input a voltage at the input (before the opamp) and the multiplexer to pass it through without much error on the other end. It should pretty much be unity when coming out the other end, as it is sensitive, voltage controlled circuitry being controlled by it and a ~2.6% difference is unacceptable (it's an entire musical note out of tune, just about)

I think I might be missing something here, but here's what I'm seeing:

I measure the potentiometer voltage divider not connected to the circuit, and it outputs 3.254V, so this is what I want to see on the output. When I connect the pot to the circuit, the actual output voltage after the CD4052 is 3.169, so has an error of about 2.5%.

Even with R1 removed, there is still over 2% error.

So, you are saying that the resistors are causing this? Even with R1 and R3 out of the circuit, it's still this way. So that's normal?

Switch internal ON resistance must be taken into consideration. It varies from 125 ohm to 1kohm depending on supply voltage. Your voltmeter must have high resistance if you want to measure correctly. It looks like it's resistance could be about 50kohms?

first of all, please use a darker font on your schematic. light grey on white is very hard to read....

second, look at the data sheets, the linearity curves show that your linearity changes with input voltage. you might think of putting buffers AFTER the switches as well, as this would minimize the errors.