The nice thing about R/2R ladders is that they only have two resistor values in them. Unfortunately, even a 12-bit R/2R DAC will require tolerances of better than 0.025% (12 bits=4096 steps, 1/4096~0.025%) if you want a monotonic output. This is the reason high-resolution DACs cost so much more than lower resolution ones. The resistors have to be laser-trimmed before the networks are encapsulated.mstechca said:After further research, it seems that the R/2R ladder is better for me, especially when dealing with 12 bits.
Here is the issue.
I have a pcb setup so that the DAC circuitry itself (just resistors, and the cmos counter (4040)) takes up more space than my regen receiver :shock:
If I removed the resistors, I would save about 2cm * 2cm of space. In fact, that is all I need if I made the super regen without any trimmers or DAC's.
I was wondering, what is the best way to minimize the required PCB space for my 12-bit DAC?
and yes, I'm using resistors as jumpers as well.
evandude said:simple answer = surface mount parts
Is there anything I can do so that resistors with 5% or 1% tolerance work?Unfortunately, even a 12-bit R/2R DAC will require tolerances of better than 0.025% (12 bits=4096 steps, 1/4096~0.025%) if you want a monotonic output.
mstechca said:Is there anything I can do so that resistors with 5% or 1% tolerance work?
like maybe changing or adding resistor values?
I'll look at programmable capacitors.
The easiest-to-understand architecture is based on a Kelvin divider. As shown in Figure 1, the output voltage is selected from the taps on a long string of equal-value resistors. Also called a string DAC, this structure has the advantage of guaranteed monotonicity. Even if one resistor were to have zero value, progressing from the bottom to the top of the divider string, the output voltage would never decrease. Monotonic means that an output level will never decrease for increasing input codes.
Search for "digital pots". Lots of companies make them. I didn't mention them because they only seem to go up to 10 bits (1024 steps), and you said you wanted 12 bits. They have internal decoders, so they will work with binary. Some are up/down pulsed units, which might be better for you.mstechca said:from further research:
Take a look at:
**broken link removed**
https://www.evaluationengineering.com states that this is a String DAC.
Check this quote out:
The easiest-to-understand architecture is based on a Kelvin divider. As shown in Figure 1, the output voltage is selected from the taps on a long string of equal-value resistors. Also called a string DAC, this structure has the advantage of guaranteed monotonicity. Even if one resistor were to have zero value, progressing from the bottom to the top of the divider string, the output voltage would never decrease. Monotonic means that an output level will never decrease for increasing input codes.
"Figure 1" is the picture above.
Does it work for binary values?
and is that site telling the truth?
mstechca said:Does it work for binary values?
and is that site telling the truth?
Yeah, but you can probably get samples of the 10 bit digital pots, and increment-decrement will work fine if you don't need random access (and you won't need a 10 bit counter!). Here is a page that lets you look at **broken link removed**.Nigel Goodwin said:mstechca said:Does it work for binary values?
No, and if you apply a couple of seconds thought to it you will soon realise why - if a couple of seconds of thought don't help, try ohms law!.
and is that site telling the truth?
Yes, but I hope you are aware that it's ONLY a 3 bit D2A (8 possible outputs) - to make a 12 bit version would require 4096 resistors, and I though you were short on space? 8)
No. They could be plus or minus 5%.mstechca said:I was wondering, could each 5% resistor (independent of manufacturer) vary as much as 2.5% above or below its value?
mstechca said:I'm wondering because I want to avoid measuring 100 resistors with an ohmmeter, and finding 12 or more that are exactly the same value.
I have a 4-bit binary-weighted DAC already working for me, but I realize that after it was built, the bits were backwards (the high bit as bit #0 instead of the low bit being bit #0).use a simple potentiometer to provide the voltage for the varicap, and proof it works, and that you have sufficient tuning range. Once that's working fine, THEN start trying to replace it with a switched solution.
mstechca said:The reason why I wanted 12 bits is because I want to be able to fine tune a station and increase the range. I am using my receiver with my transmitter. You would obviously know that if the transmitter and receiver are on the EXACT same frequency, the transmitter can transmit to the receiver better.
I agree with Nigel about the potentiometer, and I believe that a digital increment-decrement pot is mstecha's best bet eventually. While I don't agree with mstecha's approach to this, I want to point out that he doesn't have to get the resistors matched to absolute value. They only have to be matched to each other. A 4-digit multimeter should be able to do this as long as the two measured values match 2:1 including the 4th digit.Nigel Goodwin said:mstechca said:I'm wondering because I want to avoid measuring 100 resistors with an ohmmeter, and finding 12 or more that are exactly the same value.
I would suggest buying 1% resistors, as you've not much chance of matching them that closely on your meter - which is probably no better than 1% anyway, and will be at least +/1 one digit as well.
Obviously this makes it pointless building a 12 bit DAC, but 10 bits would be plenty - and having 1024 different voltages is much more than enough for the full FM band.
But I would suggest you forget the DAC altogether for now! - use a simple potentiometer to provide the voltage for the varicap, and proof it works, and that you have sufficient tuning range. Once that's working fine, THEN start trying to replace it with a switched solution.
I know. Because I am using a 2N3563 in my superregen detector portion, it only accepts a tank circuit with a 0.1 to 0.15uH inductor with a capacitor between 4 and about 15pF. If I don't meet this, then I won't get proper output. Also, since I'm wanting VHF/UHF, I won't change the transistor.I showed you before that the capacitance of a 2N4401 transistor changes 4 times as much as a 2N3904, and a circuit that uses 2N4401 transistors as varicaps that tunes from 88MHz to 92MHz.
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