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AnsweredHot!A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING

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irmak
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2019/07/12 13:42:13 (permalink)
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A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING

 
A 10K resistor is sufficient for pull up / pull down connetion of digital input channels.
 
For A/D converter input channel small pull down resistors can cause sensor overloading and it can damage PIC MCU.
 
There are different suggestions in forums 4.7K-1M.
 
http://www.resistorguide....or_pull-down-resistor/
https://electronics.stack...d-a-floating-adc-input
 
 
My personal experience on loadcell and analog encoders 10k is not sufficient.
 
How to calculate exact pull down resistor value to overcome sensor overloading ?
 

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#1
ric
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 14:33:33 (permalink)
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irmak
For A/D converter input channel small pull down resistors can cause sensor overloading

That's totally dependant upon the sensor.
If its output impedance is much more than 10k, then you need some external buffering.
 

and it can damage PIC MCU.

Who told you this rubbish?
 

My personal experience on loadcell and analog encoders 10k is not sufficient.
How to calculate exact pull down resistor value to overcome sensor overloading ?

Explain more about what sensor you want to use that requires a larger resistor?
100k should be enough, but again, if the sensor is that weak, then you're going to need a capacitor etc. to lower the AC impedance to allow the ADC to work anyway.
 

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irmak
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 15:07:29 (permalink)
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e.g Loadcell input and output terminal resistance are 350R, excitation voltage 5V, 0.002 mV/V
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NKurzman
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 16:00:41 (permalink)
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irmak
 
e.g Loadcell input and output terminal resistance are 350R, excitation voltage 5V, 0.002 mV/V


1.You are probably going to need a diff Amp for a load cell.
2. You do not need a pull-up or pull down if the input is permanently connected.
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irmak
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 22:49:04 (permalink)
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Loadcell is connected to an instrumentational amplifier. reference voltage of the instrumentational amplifer is sufficient to put output in 0-5V but when load cell is not connected to amplifier the output goes to Vss of the amplifer and this is harmfull for A/D of PIC. When I have used 10K pull down, output of amplifier stay in postive reference value but loadcell do not work properly.
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ric
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 22:54:49 (permalink)
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irmak
Loadcell is connected to an instrumentational amplifier. reference voltage of the instrumentational amplifer is sufficient to put output in 0-5V but when load cell is not connected to amplifier the output goes to Vss of the amplifer

... and you didn't think to mention these crucial details in your first post?
 
Please show us your circuit. It is a strange amplifier if just a 10k load on its output makes such a difference.
If just 10k can keep it below 5V, then it's incapable of supplying more than 0.5mA, in which case there is no way it's going to damage the PIC input.
 
 
post edited by ric - 2019/07/12 22:56:44

I also post at: PicForum
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irmak
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/12 23:10:57 (permalink)
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I have used LT1168.
 
I supply +9V, -9v, 2.5V reference.
 
In spice simulation and when loadcell is connected everything work fine. But when loadcell is not connected there is excessive negative voltage. I do not have osciloscope and I am not sure if this is a saturation or another kind of problem
 
 
I have attached the circuit in my first post
post edited by irmak - 2019/07/12 23:13:00
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/17 09:54:38 (permalink)
-1 (1)
You don't mention which PIC you are using but you should not sink current out of a pin on F1 devices.  This is negative current injection which is NEVER a good thing.  Why do you need +/-9V supply?  The LT1168 should be able to operate +5V single supply. (see typical application schematic on the LT datasheet).

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PStechPaul
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/17 18:41:24 (permalink) ☼ Best Answerby irmak 2019/07/18 10:11:52
+1 (1)
You could easily enough use something like a 1k resistor from the op-amp (instrumentation amplifier) to the PIC, which will limit the current through the protection diodes to about 4 mA if the sensor is removed and the amplifier drifts to the positive rail, and less than 10 mA if it drifts to negative rail. A 10k or even 100k resistor across the inputs of the amplifier should also work.

 
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irmak
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/18 01:21:15 (permalink)
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Many thanks for your answers.
 
1) LT1168 can be used with +5V and -5V with 2.5V ref (Output will be in 0-5V range). This is the first choice. LT1168 supply voltages should be higher than (2 Volts at least) than the pic ADC input range (0-5V). In +-5V settings LT168 saturates and edge values of sensor can not be visible.
 
2) LT1168 can also be used with +5V and grd. But one of the inputs should be 2V higher then the ground.
 
2) I have tried with pic 16f877a, PIC18f46K80
post edited by irmak - 2019/07/18 01:23:56
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/18 09:00:41 (permalink)
+2 (2)
PStechPaul
You could easily enough use something like a 1k resistor from the op-amp (instrumentation amplifier) to the PIC, which will limit the current through the protection diodes to about 4 mA <snip>




FYI Paul, 4mA of negative current injection can cause very unpredictable performance in PIC F1 devices.  Older devices were not as bad.  Negative current causes substrate current flow through the ESD diode which can change the Vf of the bandgap.   Negative current should always be clamped by a schottky diode to stay below the Vf of the ESD protection.  Positive injection is not as much of an issue.   

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NKurzman
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/18 09:19:36 (permalink)
+2 (2)
Positive Injection has its own issues.  On a Low Power Circuit it can drive Vcc over the Max.  It can bother some regulators.  And Past posters have noted the over-driving an analog pin can affect the readings on other channels.
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coffee critic
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Re: A/D PULL DOWN RESISTOR AND SENSOR OVERLOADING 2019/07/18 09:26:55 (permalink)
+2 (2)
I agree.  Positive can be a disaster to ADC readings and cause comparator mis-triggering but it generally won't misfire the BOR or shutdown INTOSC.  I generally recommend using the BAT54S to clamp both positive injection and negative injection with a single device. 

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