Thanks Lon, good starting point: directly fully proportional 0-5v output.
Lon Howze - ID escribió: > The circuit I described assumes you are only interested in the 7V to 12V > range. Also, it assumes the -V reference for the op-amp is GND. > > - Lon Howze > > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Lon Howze - ID > Sent: Thursday, March 08, 2007 10:55 AM > To: [email protected] > Subject: Re: [Sdcc-user] [Fwd: Re: [OT] - ADC Ideas] > > Two op-amps configured as a difference amplifiers with a gain of 1 could be > cascaded to provide a full-scale voltage of 5V. This would give a minimum > reading of 7V with a maximum reading of 12V. Again you would want to use the > most precise resistors and op-amps you can afford in the circuit. The 2V > input to the second op-amp could be developed from a resistor-divider network > off the 5V supply as mentioned earlier. The resistor values shown are just > one example. > > Op-Amp 1: 12V - 5V = 7V > Op-Amp 2: 7V - 2V = 5V > > > > |\ > Vin ----| \________|\ > 5V ----| / | \_____ Vout (0 - 5V) > |/ 2V |--| / > | |/ > 5V | > | | > | | > |-----| | > | 6k | | > |-----| | > |____________| > | > |-----| > | 4k | > |-----| > | > | > GND > > > > > > > > - Lon Howze > > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Markos > Sent: Thursday, March 08, 2007 10:22 AM > To: [email protected] > Subject: [Sdcc-user] [Fwd: Re: [OT] - ADC Ideas] > > Hello, > The datasheet does not mention anything about the voltage > reference max input voltage. If you see in "electrical characteristic", > states that you cannot operate the device above 0.8V over VDD in IO pins: > > VIH Input High Voltage > I/O ports: > D040 with TTL buffer 0.25 VDD + 0.8V > > Thats all i've found about it, because in the ADC chapter doesn't > mention anything about it. > > By the way: If I use two series resistor, say, I would divide the > voltage. If I divithe the voltage, say by 2, every bit in the > conversions must be muplitplied by two. > Example: > 5V Voltage reference, at 1024 bits (ADC resolution) gives me 4,8 mv /bit. > Lets imagine I connect a 10V batt with two 100k res. VIN will be 5V, > same as Vref, adc result all '1' . Here, every ADC step means 4,8 mv * 2 > (voltage divider) = 9,6 mV . Thats not accurate enough, because the PIC > (and I think any ADC) specs says +- 1 LSB in the conversion. so, the > conversion could be at least +- 9,6 mV wrong, which is not suitable for > say Nimh batts, that require the charge to be cuttof when a 10mV curve > occurs. > > So, to summarize it, If I divide the voltage, I loose resolution > (inverse proportional). Same applies to the Op Amp, which will reduce > the voltage too, but with higher part count and cost ;-) > > Regards. > > > > > > -------- Mensaje original -------- > Asunto: Re: [Sdcc-user] [OT] - ADC Ideas > Fecha: Thu, 8 Mar 2007 11:46:51 -0300 > De: C C Budden <[EMAIL PROTECTED]> > Responder a: [email protected] > Para: <[email protected]> > Referencias: <[EMAIL PROTECTED]> > > > > Hi Markos. Did Yoy take a look on the PIC datasheet? If you have a Voltage > reference (2 pins) on the IC apart power supply, and in the datasheet is > specified that can support 12 V with 5V on power supply, of course you can. > > I didn´t see that PIC datasheet, but in case that you cannot use that > voltage reference range, you will have to make a conditioning signal circuit > with an operational amplifier. > > Take care to not exed the specified ranges or damage of the device may > ocurr. > > On a simple way, if you use a couple of resistors use metal film resistor > diu to the lower inductance they have compared with others. On most AD > imputs its not recommended to use caps, inductors exept you have the > impedances well separated. > > I hope this help you. > > Regards, Casiano. > > > > > ----- Original Message ----- > From: "Markos" <[EMAIL PROTECTED]> > To: <[email protected]> > Sent: Thursday, March 08, 2007 5:48 AM > Subject: [Sdcc-user] [OT] - ADC Ideas > > > >> Hi guys, >> maybe this is a little offtopic, but i'm sure you could >> help me. I'm implementing a simple battery charger, and I need to >> messaure voltages ranges from 0 to 12 V. >> ¿Could I use the PIC integrated ADC to messaure above 5 V (provided that >> I use the voltage reference pins tied to 0 and 12 V, for example)? Most >> external ADC have a 0 to 5V input range.... >> How would you guys solve this? Any Ideas? >> >> Thanks in advance. >> >> >> ------------------------------------------------------------------------- >> Take Surveys. Earn Cash. 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