DAC's(digital to analog converters)

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DAC's(digital to analog converters)

Post by Tom » Sat Jul 06, 2002 12:31 pm

Hi, I have an interesting problem thatI have been working on and if anybody could give me any advice it would be greatly appreciated. I have a digital to analog converter and I would like to be able to drive a small galvanometer with a mirror attached to it. The galvo came from an old video laserdisc player. Right now, for testing, I have a couple of 74193 counters supplying a simple binary count to the inputs of the dac and I am using a 741 op-amp at the output of the dac. I get a nice ramp voltage on the output, but thats all I have, voltage with very little current. I need a small circuit at the output of the dac that will allow me to have both voltage and more current. I did have a simple op-amp based schematic, but I lost it! If anybody has any ideas, please let me know. Thanks, Tom.

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Re: DAC's(digital to analog converters)

Post by russlk » Sat Jul 06, 2002 1:24 pm

A 2N2222 emitter follower at the output can supply 100mA, how much current do you need?

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Re: DAC's(digital to analog converters)

Post by bobsRAC » Sat Jul 06, 2002 4:49 pm

How fast do you need the output to slew? Is this for a laser-show or some other application?<p>The uni-polar emitter follower works great for single-ended applications.
cktA<p>Do you need to both sink and source current? If so, a double-ended emitter-follower is a better solution.<p>You can construct this with a complimentary pair of npn & pnp tansistors. For low-voltage and relatively low-current a 2n3904 and 2n3906 pair works well. If you need a little more voltage, a MPSA05 and MPSA06 pair works great to 80v. ties the two emmitters together for the output, and the two bases together as the input. the npn collector is tied to the positive voltage supply and the pnp is tied to the negative voltage supply. Now, if the OP-AMP's feedback is taken from the output of the emmitter follower circuit, the two transistors give a current gain (Ai) with a near unity ( 0.99ish ) voltage gain (Av).
cktB<p>The next one will reduce distortionat the crossover point, but has the cost of added complexity.
cktC<p>This last one is mostly here for interest's sake, but if you're eager :) . This one takes a current input ( as you can see the input is clamped at one Vbe drop above V-). cktD<p>[ July 06, 2002: Message edited by: bobsRAC ]</p>

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