Microcontroller as ADC - Page 2 - Electronic Circuits Projects Diagrams Free

BOBKA · 13th March 2007, 05:09 AM

err: I meant small signal analysis

**Nigel Goodwin** · 13th March 2007, 08:37 AM

Quote:

Originally Posted by BOBKA

Guys,

let's say I want to sample voltages in a range of 40V using pic. I need to reduce the voltage to 5 volts max. You were using voltage dividers, but is there another way of doing that without making a path between Vdd and Vss? Also, according to pic datasheet, the resistance of the source should be less then 2.5k in order to sample it properly, in the case of gramo's post, is the resistance 1k||1k = 0.5k (grounding Vdd like large signal analysis for xtors)?

What's wrong with a potential divider?, it's how your meter works, it's how your scope works - there's really no better solution!.

The impedance problem is simple - check my PIC tutorials for an example, use an opamp buffer to convert the impedance to be suitable for the PIC.

gramo · 13th March 2007, 10:32 AM

So the two resistors form a parallel circuit for impedance as the PIC's capacitors charge/discharge through separate resistors?

**Nigel Goodwin** · 13th March 2007, 10:51 AM

Quote:

Originally Posted by gramo

So the two resistors form a parallel circuit for impedance as the PIC's capacitors charge/discharge through separate resistors?

Not really, it's only a crude approximation - when the input is grounded, then the two resistors are in parallel to discharge the sample and hold capacitor. But if the input is floating then it's only the one resistor. With input voltages it varies depending on the voltage and source impedance feeding the potential divider.

BOBKA · 14th March 2007, 04:08 AM

Nigel: Using voltage divider will introduce resistance to the system, and thus the system might behave in a wrong way. I will probably make a sample-and-hold block, then do the voltage divider at it's output and sample this output using pic's adc. Or I might just create a source follower, then voltage divide it and then sample this voltage using pic's adc.

In my case I just want to avoid introducing additional resistors (such as voltage dividers) to the ckt.

**Nigel Goodwin** · 14th March 2007, 08:49 AM

Quote:

Originally Posted by BOBKA

Nigel: Using voltage divider will introduce resistance to the system, and thus the system might behave in a wrong way.

Why?, you don't seem to understand what one is?, a correctly designed one causes no problems whatsoever - and they are simple to design.

Quote:

I will probably make a sample-and-hold block, then do the voltage divider at it's output and sample this output using pic's adc. Or I might just create a source follower, then voltage divide it and then sample this voltage using pic's adc.

No point whatsoever adding an external sample and hold, as it already has an internal one. Why voltage divide AFTER a buffer?, it pretty well negates the point of the buffer, and of the potential divider - check my tutorial hardware, use the buffer between the potential divider and the PIC.

Quote:

In my case I just want to avoid introducing additional resistors (such as voltage dividers) to the ckt.

Unless your input is 0-5V? (of at least 0-Vref) then there's absolutely no point, and no real alternative!.

williB · 14th March 2007, 05:16 PM

Here is Nigels anlaog input section.
http://www.winpicprog.co.uk/pic_tuto...ogue_board.htm

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13th March 2007, 05:09 AM	(permalink)
BOBKA	err: I meant small signal analysis

13th March 2007, 10:32 AM	(permalink)
gramo	So the two resistors form a parallel circuit for impedance as the PIC's capacitors charge/discharge through separate resistors? __________________ Spency. PIC Micro's - Your mind is the limit PIC's and interfacing with other devices - a PIC Basic Guide @ digital-diy.net

14th March 2007, 04:08 AM	(permalink)
BOBKA	Nigel: Using voltage divider will introduce resistance to the system, and thus the system might behave in a wrong way. I will probably make a sample-and-hold block, then do the voltage divider at it's output and sample this output using pic's adc. Or I might just create a source follower, then voltage divide it and then sample this voltage using pic's adc. In my case I just want to avoid introducing additional resistors (such as voltage dividers) to the ckt.

14th March 2007, 05:16 PM	(permalink)
williB	Here is Nigels anlaog input section. http://www.winpicprog.co.uk/pic_tuto...ogue_board.htm