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need a +-10V DAC output from the propeller. — Parallax Forums

need a +-10V DAC output from the propeller.

LawsonLawson Posts: 870
edited 2009-02-02 23:33 in Propeller 1
I'm looking for the easiest way to get a +- 10v DAC output from a propeller protoboard. The protoboard is powered by a 12v regulated power bus from the rest of the system. My current idea is filter this 12v, invert it using a switch mode inverter (capacitive or inductive), generate my DAC signal with a counter and RC filter, then run an op-amp off the +12v and -12v lines to boost the counter's output to +-10v. (assume the DAC needs to drive a ~10kohm load)

I vaguely remember a forum thread about using some propeller pins to form a small switching power supply, can't find it for the life of me.

Marty

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Comments

  • Dr_AculaDr_Acula Posts: 5,484
    edited 2009-01-31 00:27
    Yes, you can certainly get an analog output, then use an op amp to amplify it and level shift it. Eg 0 to 3V becomes -10 to +10V so a gain of 20/3 and you need to subtract 1.5V as well by setting the reference level (probably with a trim pot). Next problem is the negative voltage. You could use a 555 eg this sort of standard circuit http://www.ikalogic.com/shm_voltage_inv.php 555 chips can go up to 200mA output and this circuit can supply 80ma. If you use a prop pin instead to create the waveform you are going to need some transistors or some other technique to get the full voltage swing of 0-12V so as to generate a negative voltage. A 555 may end up simpler. But even with that circuit, I'd be concerned about switcher noise getting into the analog output. Unless you are happy with low resolution (?4-6bit) I think you will need an inverter and then a regulator. The inverter loses 1.5V and a reg maybe another 2 V and now you haven't got the full range. Plus most op amps can't swing rail to rail (eg a 324 can only get within 1.5V. Unless you use a ca 3140 or similar. So it might end up easier to build a proper negative supply eg with a mains transformer/regulator etc, or to use batteries. Can you describe the requirement a bit more eg why the need for +/-10V and is there another way of doing it and is it mains or battery powered and what sort of resolution you need?
  • LeonLeon Posts: 7,620
    edited 2009-01-31 05:56
    I'd use a proper DAC and an op-amp to generate your signal.

    Leon

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  • LawsonLawson Posts: 870
    edited 2009-02-02 21:06
    The resolution requirements are about 12-bits, more would certainly be nice. I'd like to be able to do 4000 samples a second, but realistically the seed laser this will be controlling won't respond that quickly. Both of these requirements are well within the range of what one of the counters will do in DUTY mode at 80MHz. A little noise won't be too big of an issue as the measurement of the seed laser's frequency intrinsically dithers about 1% from shot to shot. The whole system is "mains" powered.

    I did a bit more searching and found some stuff. It looks like a Microchip TC962CPA charge pump IC will generate a nice -12v rail for me. I'll use extra large capacitors to minimize ripple. Also the Linear Tech LT1498CN8#PBF rail to rail op-amp looks like it will easily do what I need.

    @Dr_Acula: I looked up the CA3140 op-amp. It doesn't look like it's a rail-to-rail op-amp. It IS a lot cheaper than the op-amp I found, know any other good op-amps too look at?

    Marty

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  • Andrew E MileskiAndrew E Mileski Posts: 77
    edited 2009-02-02 21:21
    Lawson said...
    The resolution requirements are about 12-bits, more would certainly be nice. I'd like to be able to do 4000 samples a second, but realistically the seed laser this will be controlling won't respond that quickly. Both of these requirements are well within the range of what one of the counters will do in DUTY mode at 80MHz. A little noise won't be too big of an issue as the measurement of the seed laser's frequency intrinsically dithers about 1% from shot to shot. The whole system is "mains" powered.

    I did a bit more searching and found some stuff. It looks like a Microchip TC962CPA charge pump IC will generate a nice -12v rail for me. I'll use extra large capacitors to minimize ripple. Also the Linear Tech LT1498CN8#PBF rail to rail op-amp looks like it will easily do what I need.

    @Dr_Acula: I looked up the CA3140 op-amp. It doesn't look like it's a rail-to-rail op-amp. It IS a lot cheaper than the op-amp I found, know any other good op-amps too look at?

    Marty
    So many to choose from. How about NJU7031D-ND, 8-pin DIP, $0.75 on Digi-Key.
  • LawsonLawson Posts: 870
    edited 2009-02-02 23:09
    The NJR NJU7031D is a nice looking rail to rail op-amp but it doesn't look like it'd be too happy with a ±12 volt supply.

    Lawson

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  • Andrew E MileskiAndrew E Mileski Posts: 77
    edited 2009-02-02 23:33
    Lawson said...
    The NJR NJU7031D is a nice looking rail to rail op-amp but it doesn't look like it'd be too happy with a ±12 volt supply.

    Lawson
    Sorry, I only saw 10 Volt mentioned. You can probably do a parametric search yourself, as I obviously can't read blush.gif
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