Post by thread:D/A with an f84

The better choice, i think, is to use the AD558 (ANALOG DEVICES)... Works with 5 Volts power supply... Miguel Andrew Kelley wrote: {Quote hidden}> > Does anyone know how to make an analog voltage/signal with an f84? > > If so, please let me know. (Duh =) > > TIA, > Andrew > ________________________________________________________________ > YOU'RE PAYING TOO MUCH FOR THE INTERNET! > Juno now offers FREE Internet Access! > Try it today - there's no risk! For your FREE software, visit: > dl.http://www.juno.com/get/tagj

On Fri, 17 Mar 2000, Andrew Kelley wrote: > Does anyone know how to make an analog voltage/signal with an f84? This will make a DC signal on the lsb of porta: bsf porta,0 as will this: bcf porta,0 A "strategic" combination of these can be concatenated to produce a waveform whose average value can be used to produce an analog voltage. Or as we prefer to say on the list every few days: PWM. Here's a software pwm that works on the f84: http://www.dattalo.com/technical/software/pic/pwm256.txt But it's probably overkill for what you want to do. But, OTOH, I have no idea what you exactly want...

It is not critical at all. I just want to be able to control the speed of a motor like slow medium or fast. Andrew On Fri, 17 Mar 2000 12:06:17 -0600 Scott Dattalo <spam_OUTscottTakeThisOuTDATTALO.COM> writes: {Quote hidden}> On Fri, 17 Mar 2000, Andrew Kelley wrote: > > > Does anyone know how to make an analog voltage/signal with an f84? > > > This will make a DC signal on the lsb of porta: > > bsf porta,0 > > as will this: > > bcf porta,0 > > A "strategic" combination of these can be concatenated to produce a > waveform > whose average value can be used to produce an analog voltage. > > Or as we prefer to say on the list every few days: PWM. Here's a > software pwm > that works on the f84: > > http://www.dattalo.com/technical/software/pic/pwm256.txt > > > But it's probably overkill for what you want to do. But, OTOH, I > have no idea > what you exactly want... ________________________________________________________________ YOU'RE PAYING TOO MUCH FOR THE INTERNET! Juno now offers FREE Internet Access! Try it today - there's no risk! For your FREE software, visit: dl.http://www.juno.com/get/tagj.

An R-2R ladder can be used to turn a group of PIC output pins into a DAC for a few cents. Below is a 3-bit DAC. Using 3 PIC output pins (beware of the odball open drain pins, they're no good for this), this circuit will provide 0 - Vcc*(7/8) volts out, with a 2R source impedence. It can be extended to more bits by replicating the MSB R-2R end of the ladder. Up to 4 bits can be achieved with 5% resistors. Up to 6 bits can be achieved with 1% resistors. I know this works because I've used it. R=10K works fine. Up to 10 bits might be achieved with .1% resistors. Beware, this circuit uses Vcc as its reference, so you would have to use a precision reference for your Vcc to achieve this accuracy, and I would still figure it was doubtful due to FET on-resistance, digital noise, etc. Besides, .1% resistors are pricey enough that you still might be ahead to buy a DAC in a chip. In many applications, it would be advisable to buffer the output with an op-amp. A circuit like this can achieve waaay higher output frequencies than PWM, and can be easier to filter, at the expense of using up a bunch of PIC pins. OUT o--+-/\/\/\/\----o MSB | 2R \ / \ R / | +-/\/\/\/\----o Middle bit | 2R \ / R \ / +-/\/\/\/\----o LSB | 2R \ / \ 2R / | --- /// > {Original Message removed}

There's alot of ways to do what you want, but I've been playing with a Voltage-output 8-bit digital-to-analog converter "AD558" from Analog Devices. You can find the specs at; products.analog.com/products/info.asp?product=AD558 This is a parallel control input or you can use a "serial to parallel" shifter from the pick and not tie up so many ports. This chip can output either 0-2.56 +vdc or 0-10 +vdc with 8 bit resolution. It works great or you can use PWM, if you don't need the pic to do much more than that. {Quote hidden}> It is not critical at all. I just want to be able to control the speed of > a motor like slow medium or fast. > > > Andrew > > On Fri, 17 Mar 2000 12:06:17 -0600 Scott Dattalo <.....scottKILLspam@spam@DATTALO.COM> > writes: > > On Fri, 17 Mar 2000, Andrew Kelley wrote: > > > > > Does anyone know how to make an analog voltage/signal with an f84? > > > > > > This will make a DC signal on the lsb of porta: > > > > bsf porta,0 > > > > as will this: > > > > bcf porta,0 > > > > A "strategic" combination of these can be concatenated to produce a > > waveform whose average value can be used to produce an analog voltage. > > > > Or as we prefer to say on the list every few days: PWM. Here's a > > software pwm > > that works on the f84: > > > > www.dattalo.com/technical/software/pic/pwm256.txt > > > > > > But it's probably overkill for what you want to do. But, OTOH, I > > have no idea > > what you exactly want... > > ________________________________________________________________ > YOU'RE PAYING TOO MUCH FOR THE INTERNET! > Juno now offers FREE Internet Access! > Try it today - there's no risk! For your FREE software, visit: > dl.http://www.juno.com/get/tagj. > >

What do you mean by 2r 2 times r? Andrew On Fri, 17 Mar 2000 13:44:44 -0600 Don Hyde <DonHKILLspamAXONN.COM> writes: {Quote hidden}> An R-2R ladder can be used to turn a group of PIC output pins into a > DAC for > a few cents. Below is a 3-bit DAC. Using 3 PIC output pins (beware > of the > odball open drain pins, they're no good for this), this circuit will > provide > 0 - Vcc*(7/8) volts out, with a 2R source impedence. > > It can be extended to more bits by replicating the MSB R-2R end of > the > ladder. > > Up to 4 bits can be achieved with 5% resistors. > > Up to 6 bits can be achieved with 1% resistors. I know this works > because > I've used it. R=10K works fine. > > Up to 10 bits might be achieved with .1% resistors. Beware, this > circuit > uses Vcc as its reference, so you would have to use a precision > reference > for your Vcc to achieve this accuracy, and I would still figure it > was > doubtful due to FET on-resistance, digital noise, etc. Besides, .1% > resistors are pricey enough that you still might be ahead to buy a > DAC in a > chip. > > In many applications, it would be advisable to buffer the output > with an > op-amp. > > A circuit like this can achieve waaay higher output frequencies than > PWM, > and can be easier to filter, at the expense of using up a bunch of > PIC pins. > > OUT o--+-/\/\/\/\----o MSB > | 2R > \ > / > \ R > / > | > +-/\/\/\/\----o Middle bit > | 2R > \ > / R > \ > / > +-/\/\/\/\----o LSB > | 2R > \ > / > \ 2R > / > | > --- > /// > > > {Original Message removed}

> > > > In many applications, it would be advisable to buffer the output > > with an > > op-amp. > > In fact, doesn't an R-2R D/A have a CURRENT output, requiring an op-amp current to voltage converter? Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ YOU'RE PAYING TOO MUCH FOR THE INTERNET! Juno now offers FREE Internet Access! Try it today - there's no risk! For your FREE software, visit: dl.http://www.juno.com/get/tagj.

Coincedently, I was thinking about resister ladders this morning. I'm wondering about the circuit below, doesn't the output vary with the impedance of the device OUT is going to? If the LSB is set, then at one point in the circuit you can depend on 2.5 volts (assuming Vdd =5v), but there are two R between that point and out, and the point OUT would have different voltage depending on the device's impedance that you have hooked up to out. Am I looking at this the wrong way? -Adam Don Hyde wrote: {Quote hidden}> OUT o--+-/\/\/\/\----o MSB > | 2R > \ > / > \ R > / > | > +-/\/\/\/\----o Middle bit > | 2R > \ > / R > \ > / > +-/\/\/\/\----o LSB > | 2R > \ > / > \ 2R > / > | > --- > /// > > > {Original Message removed}

On Fri, Mar 17, 2000 at 12:58:16PM -0800, Harold M Hallikainen wrote: > > > > > > In many applications, it would be advisable to buffer the output > > > with an > > > op-amp. > > > > > In fact, doesn't an R-2R D/A have a CURRENT output, requiring an op-amp > current to voltage converter? No. If you build it as shown in the "ASCII art" below. Just consider the digital ouputs to be switched voltage sources with Vdd or 0 output voltage (it is almost true for CMOS outputs). Then the whole circuit is equivalent to the circuit shown in the right column below, where Ux is equal: -1 -2 -n+1 -n -n-1 Ux=Vcc*(Q 2 +Q 2 +...Q 2 +Q 2 +Q 2 ) n n-1 2 1 0 So there is no need for I/U converter, however it may be used. If smaller output resistance is required, the output voltage follower or amplifier (eg. OA based) should be used. GND-|2R|--+ | +--|R|-- Output Q0 -|2R|--+ _| R / \ Q1 -|2R|--+ is equivalent to: |Ux| R \__/ Q2 -|2R|--+ | R GND . . . . . . R Qn-1-|2R|-+ R Qn -|2R|--+-- Output -- Wojciech Zabolotny http://www.ise.pw.edu.pl/~wzab http://www.freedos.org Free DOS for free people!

{Quote hidden}> -----Original Message----- > From: Harold M Hallikainen [.....haroldhallikainenKILLspam.....JUNO.COM] > Sent: Friday, March 17, 2000 2:58 PM > To: EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU > Subject: Re: D/A with an f84 > > > > > > > > In many applications, it would be advisable to buffer the output > > > with an > > > op-amp. > > > > > In fact, doesn't an R-2R D/A have a CURRENT output, > requiring an op-amp > current to voltage converter? > > Harold > > > > FCC Rules Online at http://hallikainen.com/FccRules > Lighting control for theatre and television at > http://www.dovesystems.com > > ________________________________________________________________ > YOU'RE PAYING TOO MUCH FOR THE INTERNET! > Juno now offers FREE Internet Access! > Try it today - there's no risk! For your FREE software, visit: > dl.http://www.juno.com/get/tagj. >

On Fri, 17 Mar 2000 23:08:30 +0100 Wojciech Zabolotny <wzabspam_OUTISE.PW.EDU.PL> writes: > > > In fact, doesn't an R-2R D/A have a CURRENT output, > requiring an op-amp > > current to voltage converter? > > No. If you build it as shown in the "ASCII art" below. Just consider > the > digital ouputs to be switched voltage sources with Vdd or 0 output > voltage > (it is almost true for CMOS outputs). Well... now I've gotta check this out. So, let's see how it goes. My first ever ascii art of a 2 bit (25 cent?) R-2R D/A is below: R4 1K +----------+--/\/\----+----------out | | | / / / \ 2K \ 2K \ 2K / R1 / R2 / R3 \ \ \ | | | | | | gnd B0 B1 Let's assume B0 and B1 can be either 0V or +5V, representing 0 and 1. Further, B0 is the LSB. We'll analyze a few cases, then apply superposition for the remaining case. Case 0 - B0=B1=0 (0V) The output is 0V (not much analysis here). Case 1 - B0=1 (+5V), B1=0 (0V) Thevenize R1, R2, and B0 to get +2.5V behind 1K. Combine above with R4 to the +2.5V behind 2K. Since B1 is at 0V (ground), the above Thevenin equivalent and R3 form a voltage divider, leaving the output as 1.25V behind 1K. Case 2 - B1=1 (+5V), B0=0 (0V) R1 and R2 are now in parallel, giving 1K, which is in series with R4, giving 2K to ground for the left side of the circuit. That, along with R3 form a voltage divider making the output +2.5V behind 1K. Case 3 - B0=B1=1 (+5V) Using superposition, we can just add the voltages above. The output resistance remains the same. We end up with +3.75V behind 1K. So... I STAND CORRECTED!!! The R-2R D/A does linearly output voltage and does not require a current to voltage converter. Further, the output resistance of this D/A is constant, so the variable voltage behind a fixed resistance is ALSO appropriate fro driving a current to voltage converter. Thanks! Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ YOU'RE PAYING TOO MUCH FOR THE INTERNET! Juno now offers FREE Internet Access! Try it today - there's no risk! For your FREE software, visit: dl.http://www.juno.com/get/tagj.