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In an R-2R ladder based on standard 5% resistors, which resistor would be best to add a trim pot to adjust/tune the output for each bit? R or 2R?

#### Pommie

##### Well-Known Member
Because of the way resistors are graded, 5% resistors will be either 5% higher or 5% lower. Therefore, if you meter them and only use one of the two groups, you should get a pretty good result without tuning.

Mike.

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OK. So it was late when I made that post. Duh on me.

#### AnalogKid

##### Well-Known Member
5% is one part in 20, or an ENOB (effective number of bits) of 4. Without hand selection of the resistors, any ladder larger than 4 stages will deliver only 4 bits of accuracy.

ak

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From my experience on another project, there are not many R and 2R values of resistors available. So the easiest thing is to physically use R value resistors and 2 R in series (or make a 1/2R-R ladder and use 2 in parallel and a single.)

What value resistor is typically used? Specifically, I will be using 20V in for one ladder and 8V in for another ladder, because the possible values out are what I'm looking for.

#### ronsimpson

##### Well-Known Member
I will be using 20V in for one ladder and 8V in for another ladder
What is your output voltage range? 0 to 10V?
I use a 0 to 5V ADC and an amplifier to extend that to what I want. (0 to 24V)

##### Member
I need very specific 1.00v increments. 20V will be broke in 5V increments and the 8V broke into 1V increments, then added to provide the 1.00V increments.

#### ronsimpson

##### Well-Known Member
What is driving the R-2R? A micro computer?

##### Member
A comparator circuit that needs 1.00V increment thresholds.

I'm trying to decide if I'm better off with 100k or 10k resistors, or something in between. The output will be opamp unity gain buffered.

#### ronsimpson

##### Well-Known Member
I think you have a unknown voltage in the range of 0 to 20V.
You want to make a "reference" voltage of 0,1,2,3,....18,19,20V
A comparator will compare the two voltages.
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What about using a voltage divider to make the unknown voltage (0 to 20V) make it smaller like (0 to 5V)?
Now your R-2R thing can happen at a more normal 5V.

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Ron, I'm not sure where you are going, but it's not in the direction to output the voltages I need.

Question is what values of resistors would you use for an R-2R ladder with a 20V input voltage and for a ladder with an 8V input voltage?

The 20V R-2R ladder will output 0-5-10-15 volts and the 8V ladder will output 1-2-3-4-5-6-7 volts. The relays are driven by a rotary switch and the outputs feed to a unity gain buffer opamp.

#### ronsimpson

##### Well-Known Member
So you have a 4 position rotary switch and a 7 position rotary switch and a 20V supply.
You could dump the relays and r-2r and just stack the resistors. (example using a 10 position switch)

##### Member
But a computer cannot control a rotary switch.

Does the actual value of resistors in an R2-R ladder matter, or is 4.7K, 10K, and 100K all just as good given the load will feed an opamp buffer so there is no worry as to the load outside the ladder?

#### ronsimpson

##### Well-Known Member
I would choose 10k, not critical. You don't want to heat the resistor much. (example 100 ohms) The buffer will have a high input impedance so it will be 100s of time higher than the R2-R. Hope this helps.

The relays are driven by a rotary switch and the outputs feed to a unity gain buffer opamp.
But a computer cannot control a rotary switch.
Much confused. Don't try to explain now. Moving on.

##### Member
Same here. Only took 5 posts to get an expert opinion on a fairly straightforward question. Even better, the entire design (almost project) has been changed. Back to the starting board...

#### AnalogKid

##### Well-Known Member
Same here. Only took 5 posts .
Really? Based on your last seven posts I thought it would take at least another five or six to pry loose from you ... forget it.

##### Member
Really? Based on your last seven posts I thought it would take at least another five or six to pry loose from you ... forget it.
I tried to keep my questions short, direct, and to the point. I don't know how else I could have done better. I'm not sure what else you all would want.

#### AnalogKid

##### Well-Known Member
Context. The experience difference between a 20-year designer and a 50-year designer breeds a healthy respect for context. I'm a big fan of answering the question that was asked, but over 50% of the time it turns out that that isn't the real question.

What are the switches driving the ladder inputs? Electronic or mechanical?

Resistance/impedance? Constant with voltage and/or temperature?

What is the input impedance of whatever the ladders drive? Does the ladder impedance have to be kept low to swamp out local noise sources?

How solid/stable are the 8 V and 20 V sources? Accuracy? Drift? Noise? Output impedance over temperature?

How fast are the inputs changing?

Are the resistors thru-hole or SMT? What is the power rating?

For the same resistor values in both ladders, there is a 6.25x difference in power dissipation. Is this important?

What is the power budget? Is this battery operated? If so, what are the expected circuit current and operating time?

In post #7 you said "I need very specific 1.00v increments." That implies at least 8-bit accuracy so you can tell the difference between 0.99, 1.00, and 1.01. If you mean to extend this to differentiate between 19.00 and 19.01, that is 12 bit accuracy, requiring 0.02% tolerance resistors.

AND - what is with the two voltages and combining the outputs of the two ladders? Approximately zero percent of that is clear? Does this mean that the impedances of the two ladders interact? Does one ladder have to be at least 10 times the impedance of the other to minimize loading?

Context.

ak

Just edited - reload for more brilliance

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Keep in mind I will find out soon what the next generation of the design will be. But I'm interested in learning about the rest of this design.

Context. The experience difference between a 20-year designer and a 50-year designer breeds a healthy respect for context. I'm a big fan of answering the question that was asked, but over 50% of the time it turns out that that isn't the real question.
I have noticed that. One reason why I'm not usually the first responder.

What are the switches driving the ladder inputs? Electronic or mechanical?
They were mechanical. Either relay or rotary switch.

How solid/stable are the 8 V and 20 V sources? Accuracy? Drift? Noise? Output impedance over temperature?
How accurate and stable? I hope enough.

Resistance/impedance? Constant with voltage and/or temperature?
Should be, indoor lab use only.

What is the input impedance of whatever the ladders drive? Does the ladder impedance have to be kept low to swamp out local noise sources?
opamps, so high impedance.

How fast are the inputs changing?
VERY VERY Slowly. manually input setting.

Are the resistors thru-hole or SMT? What is the power rating?
I was taught/told years ago to select resistor power based on 5x the typical design power rating. Under the premise, the higher the difference in power rating (actual vs design) the more temperature stable the value will be. Obviously there is a trade off of stability vs size. You don't see 10W resistors on every board.

For the same resistor values in both ladders, there is a 6.25x difference in power dissipation. Is this important?
Not unless it effects the how/why of the resistor value(s) selected.

What is the power budget? Is this battery operated? If so, what are the expected circuit current and operating time?
Wall powered, lots of juice available.

In post #7 you said "I need very specific 1.00v increments." That implies at least 8-bit accuracy so you can tell the difference between 0.99, 1.00, and 1.01. If you mean to extend this to differentiate between 19.00 and 19.01, that is 12 bit accuracy, requiring 0.02% tolerance resistors.
My point there was that I needed 1.0V increments, only 1.0V increments, and not some binary DAC represesentation close to a 1V increment.

I was looking for the best to get 1.00V increments, in the least number of components with values closest to 1.00V. The other options I was considering had substantial error in the required vs realized voltages. A 12 bit 0-5V DAC amplified 4x resulted in some values off by more than 50mV with a lot of options I didn't need (only 20 of 4096 values would be used). My thought was I could select a set of resistors in a R2R ladder that would give me exactly only the values needed. The combination of the two ladders produces only the 20 1.00V options required.

AND - what is with the two voltages and combining the outputs of the two ladders? Approximately zero percent of that is clear? Does this mean that the impedances of the two ladders interact? Does one ladder have to be at least 10 times the impedance of the other to minimize loading?
I did not feel that was relevant. Why does it matter what I'm doing with them? I have found I have to keep the post short and isolated to the point on hand. Otherwise the thread devolves into design by committee (see above) or I never get an answer to the question at hand (example present). I am looking for the methodology on selecting the value of R best suited for an R-2R ladder that will be used to breakdown a 20V input and an 8V input. Furthermore, each ladder was to be opamp buffered due to the difference in source and, possible, output impedances.

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#### AnalogKid

##### Well-Known Member
A 12 bit 0-5V DAC amplified 4x resulted in some values off by more than 50mV
5 / 4095 = 0.00122 V
0.00122 x 4 = 0.00488 V -- less than 5 mV +/- the accuracy of the 5 V

ak

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