More Fun with D/A Converters

Transcription

1 D/A with a PIC Miroontrollr (rap) Mor Fun with D/A Convrtrs A subroutin to driv this hip follows: void D2A(unsignd int ) { unsignd har i; TRISC0 = 0; TRISC3 = 0; TRISC5 = 0; // Add 0011 to th first four bits to st up th D/A = & 0x0FFF; = + 0x3000; RC0 = 1; RC3 = 1; // CS gos low to slt th D/A hip RC0 = 0; // Snd out 16 bits of data for (i=0; i<16; i++) { if ( & 0x8000) RC5 = 1; ls RC5 = 0; RC3 = 0; = << 1; RC3 = 1; } // CS gos high to trminat th ommuniaitons RC0 = 1; } JSG - 1 -

2 DC Motor Drivr: Driv a DC Motor at 0..20V (Voltag Control) With a D/A, a PIC an output with 4095 stps. With this, you an driv a DC motor at 0..20V with 4095 stps. Hardwar: To amplify th voltag 4x ( boms 0..20V), us a non-invrting amplifir. To amplify th urrnt, us a powr transistor (TIP112) With ngativ fdbak, V+ quals V-. For this to happn, th output must b 4x th input. Th op-amp's output is whatvr it taks to mak V+ qual to V-. With th 1.4V drop aross th transistor's bas to mittr juntion, this rsults in th op-amp outputting 4 tims th input, plus 1.4V (not that w rally ar - but it dos). Th transistor provids a urrnt gain of This mans that to driv a load of 3A, th op-amp only nds to sour 3mA - somthing an LM833 is apabl of doing. 22V D2A Output LM833 Y + 1.4V 4 Ib Y = 4 b I TIP112 NPN 3A max 0..20V 3k M 1k With this iruit, you an vary th voltag to th motor (and hn th spd) from 0V to 20V with 4095 stps using a PIC miroontrollr. JSG - 2 -

3 LED Drivr: Driv an LED at mA (Currnt Control) A slight variation on this iruit is a urrnt amplifir. Again, an op-amp with ngativ fdbak to for V+ to qual V-. To ontrol urrnt, pla a rsistor in sris with th diod. Add a rsistor to st th maximum urrnt (100mA) whn is its maximum voltag (5V) R 5V 100mA 50 Add a transistor to amplify th urrnt from th op-amp. Using a TIP112 (with a urrnt gain of 1000) rsults in th op-amp having to output 0.1mA whn driving 100mA to th load - somthing an LM833 is apabl of doing. Pla th diod in sris with th 50 Ohm rsistor so that any urrnt going through R also gos through th diod (almost: th bas urrnt splits of 1/1000th of th urrnt to th 50 ohm rsistor). 10V 99.9mA + 3V From D/A 5V LM Vb 0.1mA 7V - TIP112 NPN 5V 100mA 50 With this iruit, you an ontrol th urrnt to th LED from 0 to 100mA with 4095 stps. Also not: if you rpla th diod with a DC motor, you'r ontrolling th urrnt to th motor (torqu) rathr than voltag (spd). Also also: Bak in EE 206, you usd urrnt sours. That's what this iruit is: th urrnt to th load is st by th voltag at. JSG - 3 -

4 DC Motor Drivr: Driv a Spakr (or DC Motor) at -10V to +10V If you want to apply positiv and ngativ voltags to th load, you first nd to amplify th output of th D/A onvrtr () so that it's V. To do this, us an instrumntation amplifir: from D/A Y V 2.5V On you hav a V signal, driv a push-pull amplifir +15V from D/A Y +10V +11.4V b 1.25mA TIP112 NPN 1.25A Push -10V +10V 2.5V +10V -10V b -11.4V 1.25mA -10V 1.25A Pull TIP117 PNP 8 Ohm Spakr -15V With this, you an now output -10V.. +10V to th load, Driving a spakr forward and bak, or Driving a motor forward or in rvrs With 4095 stps JSG - 4 -

5 Eltroni Sin Wav Gnrator So far, w'v bn driving a spakr with a squar wav whn making an ltroni piano. This rsults in a harsh buzzing sound. For a lan sound, output a sin wav. To do this, first, approximat a sin wav with N data points. Assum for xampl w want th sin wav to go from -2V to +2V. Using th prvious iruit Numbr to D/A D/A Output Ciruit Output 0 0V -10V 4,095 5V +10V Th output voltag is rlatd to th numbr you snd to th D/A as Volts D/A V 10V To output -2V, you nd to snd th numbr 1638 To output +2V, you nd to snd th numbr 2457 Lt's us 16 points to approximat a sin wav. Us Matlab to gnrat a sin wav whih gos from 1638 to >t = [1:16]' / 16 * 2 * pi; -->y = sin(t) * ; -->y = round(y); This looks lik th following whn you snd ths voltags to th D/A JSG - 5 -

6 It's not a prft sin wav. To do bttr, mor points would hlp - but mor points mans th PIC will hav to output numbrs to th D/A that muh fastr. To play a 220Hz sin wav, On yl = 1/220 sond (4.54ms) Thr ar 16 D/A alls pr yl (w'r rprsnting a sin wav with 16 points) Eah D/A all = 286.1us From bfor, whn w wrot a wait routin, w found that ounting to 620 with an intgr taks about 1ms us should rquir ounting to 177 N s 1ms Th following od should gnrat a 220Hz sin wav: onst unsignd int TABLE[16] = [2204, 2336, 2425, 2456, 2425, 2336, 2204, 2047, 1890, 1758, 1669, 1638, 1669, 1758, 1890, 2047]; // in th main routin unsignd int i, j; whil(1) { i = (i + 1) % 16; D2A(TABLE[i]); for(j=0; j<177; j++); } JSG - 6 -

7 Eltroni Tuba Not that if you hang th look-up tabl, you hang th signal snt to th spakr. Th signal dtrmins what typ of instrumnt you'r mimiking: JSG - 7 -