How to Select Resistor Values for R-2R Ladder DAC with 13kΩ Op Amp Output Resistance

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#1 21665216 11 Dec 2012 04:48

chanti Reddy chanti Reddy

Anonymous

Post #1
21665216 11 Dec 2012 04:48

how to select Resistor value for R-2R ladder dac.
my op amp is having 13k ohms output resistance.
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#2 21665217 11 Dec 2012 05:10

Mohammad Honarparvar Mohammad Honarparvar

Anonymous

Post #2
21665217 11 Dec 2012 05:10

Would you please let me know about the values of feeback resistor and reference voltage?
#3 21665218 11 Dec 2012 15:48

Earl Albin Earl Albin

Anonymous

Post #3
21665218 11 Dec 2012 15:48

Do you have a schematic? Usually the resistors are set such that it divide the reference voltage by a bit count. Example say you have 5 bits, the reference voltage is divides by the following:

1) Vref/1
2) Vref/2
3) Vref/4
4) Vref/8
5) Vref/16

So that when you select you go from full voltage down to 1/16 full voltage if of course it's say 5 bits. So if you do a quick check on your divide string, what does it give you?
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#4 21665219 12 Dec 2012 14:30

Steve Lawson Steve Lawson

Anonymous

Post #4
21665219 12 Dec 2012 14:30

Oops, misread your reply...sorry.
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#5 21665220 12 Dec 2012 14:32

Steve Lawson Steve Lawson

Anonymous

Post #5
21665220 12 Dec 2012 14:32

That seems rather high for a output impedance -- or do you mean that you have a 13k resistor on the output -- what? As a load or as a feedback resistance or ???
#6 21665221 22 Dec 2012 01:23

chanti Reddy chanti Reddy

Anonymous

Post #6
21665221 22 Dec 2012 01:23

sir, here out put impedence means op amp with out put buffer output impedence.no extra 13k resistor is there
#7 21665222 22 Dec 2012 04:50

Earl Albin Earl Albin

Anonymous

Post #7
21665222 22 Dec 2012 04:50

Why not go and buy a college book that covers this subject and look at how a design example is done 1st. Then submit a schematic. You ask a design question and you submit no schematic? Why?
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#8 21665223 22 Dec 2012 07:00

chanti Reddy chanti Reddy

Anonymous

Post #8
21665223 22 Dec 2012 07:00

sir, i studied books namely :data converters by franco maloberti,roy choudary, pipelined adc design by imran ahmed.but i didnot get these details i.e., how to select resistor values .please suggest a reference for these design examples. schematic r-2r ladder dac only.
#9 21665224 22 Dec 2012 07:02

chanti Reddy chanti Reddy

Anonymous

Post #9
21665224 22 Dec 2012 07:02

sir, those values i have to find out. please suggest an idea to get those values
#10 21665225 23 Dec 2012 15:01

Earl Albin Earl Albin

Anonymous

Post #10
21665225 23 Dec 2012 15:01

Here is what the switches do. Ground all and the input goes to zero, unground all and connect to Vref, and Vout should approach 2x Vref. So as to what you want Vref to be that depends, if you want the output to represent the original sampled voltage, then Vref would be Vmax (sampled)/2.

Say you sampled some voltage and the max sampled voltage was 5V, the Vref would be 5V/2. Therefore when you set all the switched to Vref, the output would approach 5V (never actually reached 5V because of resolution).

Have you actually written out a table that shows what the output voltage is as a function of the switch settings? I hope so. I don't know where this particular algorithm came from but that doesn't matter, it has a purpose.

Have you sorted out any of this, I mean how it works as designed? If so is that how you want it to work?
#11 21665226 26 Dec 2012 04:50

Steve Lawson Steve Lawson

Anonymous

Post #11
21665226 26 Dec 2012 04:50

What is the part number of this op-amp?
#12 21665227 27 Dec 2012 11:14

Mark Harrington Mark Harrington

Anonymous

Post #12
21665227 27 Dec 2012 11:14

Hi

Lots of information here explaining DAC design with schematic examples etc including spdif

see upload below
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Topic summary

✨ The discussion addresses selecting resistor values for an R-2R ladder DAC when the associated op amp has a high output impedance of approximately 13kΩ. Clarification was sought regarding the feedback resistor and reference voltage values, as well as the presence of a schematic. It was noted that the 13kΩ output impedance refers to the op amp's internal output buffer impedance, not an external resistor. The resistor values in an R-2R ladder DAC are typically chosen to create binary-weighted voltage divisions of the reference voltage, such as Vref/1, Vref/2, Vref/4, etc., corresponding to the number of bits. The output voltage ideally ranges from zero to nearly twice the reference voltage, depending on switch settings. The reference voltage selection depends on the maximum sampled input voltage, often set to half the maximum to allow full-scale output swing. The importance of writing out a truth table for switch settings versus output voltage was emphasized. Recommendations included consulting detailed DAC design literature and providing a schematic for more precise guidance. Additional resources with schematic examples and design explanations were suggested.
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FAQ

TL;DR: In this thread, full‑scale from an R‑2R DAC “should approach 2x Vref,” a concrete 2× statistic you can design around. “Vout should approach 2x Vref.” [Elektroda, Earl Albin, post #21665225]

Why it matters: If you’re asking “how do I pick R/2R and Vref with a 13kΩ op‑amp output?,” this FAQ distills the practical choices for students and engineers building or debugging an R‑2R DAC front end.

Quick Facts

Full‑scale output target: approaches 2×Vref in the discussed inverting-summing setup. [Elektroda, Earl Albin, post #21665225]
Bit weights follow powers of two: Vref/1, Vref/2, Vref/4, … for N bits. [Elektroda, Earl Albin, post #21665218]
For a 0–5 V sampled signal, set Vref = Vmax/2 = 2.5 V. [Elektroda, Earl Albin, post #21665225]
13 kΩ reported as op‑amp output impedance; another poster calls that “rather high.” [Elektroda, Steve Lawson, post #21665220]
Feedback resistor value and Vref must be specified to size the ladder correctly. [Elektroda, Mohammad Honarparvar, post #21665217]

How do I pick R and 2R if my op‑amp output resistance is 13 ka?

First confirm what the 13 kΩ means: true output impedance, a load, or a feedback part. One poster flags 13 kΩ as unusually high for an op‑amp output. Clarify the part number and schematic before sizing the ladder, since the answer depends on topology and target gain. [Elektroda, Steve Lawson, post #21665220]

What does full‑scale output equal in this R‑2R DAC setup?

With all switches tied to Vref in the inverting-summing configuration described, the output should approach 2×Vref. “Vout should approach 2x Vref.” This is the key sizing anchor for your reference selection and gain budgeting. [Elektroda, Earl Albin, post #21665225]

How should I choose Vref to reconstruct a 0–5 V sampled signal?

Use Vref = Vmax/2. For a 0–5 V range, pick Vref = 2.5 V. With all bits on, the output then approaches 5 V, acknowledging finite resolution means it never reaches exactly 5.0 V. [Elektroda, Earl Albin, post #21665225]

How are bit weights implemented in an R‑2R ladder?

Each less significant bit contributes half the previous one: Vref/1, Vref/2, Vref/4, Vref/8, and so on. For five bits, the smallest weight is 1/16 of the reference contribution. This binary weighting is the ladder’s defining property. [Elektroda, Earl Albin, post #21665218]

Do I need a schematic to get useful resistor values?

Yes. A responder explicitly asked for a schematic and suggested reviewing textbook examples before sizing parts. Without a schematic and gain target, feedback and ladder resistor values remain undefined. [Elektroda, Earl Albin, post #21665222]

What feedback resistor (Rf) should I use in the op‑amp DAC stage?

The exact Rf depends on your intended transimpedance gain and reference scaling. In the thread, a participant asked you to specify Rf and Vref because they determine the design. Provide these to move from concept to numbers. [Elektroda, Mohammad Honarparvar, post #21665217]

What happens when all ladder switches connect to ground or to Vref?

All grounded yields zero output. All tied to Vref makes the op‑amp output approach 2×Vref. This behavior is a quick sanity check for your build and simulations. [Elektroda, Earl Albin, post #21665225]

Why wonct my output hit the exact top code value?

Finite resolution prevents reaching the exact rail. With the given example, setting all bits on approaches the target (for 5 V full‑scale) but never reaches exactly 5.000 V. That’s inherent to quantization. [Elektroda, Earl Albin, post #21665225]

Is 13 ka output impedance acceptable for driving an R‑2R ladder?

A commenter called 13 kΩ “rather high” and asked whether it was a real output impedance or just a resistor on the output path. Verify the parameter from the op‑amp datasheet and share the part number. [Elektroda, Steve Lawson, post #21665220]

Do I need to share the op‑amp part number?

Yes. Another participant specifically asked for the op‑amp’s part number. Device output stage and drive capability influence ladder sizing, settling, and error. [Elektroda, Steve Lawson, post #21665226]

Where can I see complete DAC design examples?

A forum member pointed to an uploaded resource with DAC design explanations and schematics, including SPDIF‑related content. Check the referenced upload under that post. [Elektroda, Mark Harrington, post #21665227]

What is an R‑2R ladder DAC in plain terms?

It’s a resistor network where each bit contributes a voltage that halves per step: Vref/1, /2, /4, /8, … Summed through an op‑amp, these weighted contributions form the digital‑to‑analog output. [Elektroda, Earl Albin, post #21665218]

Quick how‑to: sanity‑check my ladder math in 3 steps?

Choose Vref = Vmax/2 for your target output span.
Ground all switches and verify the output is near zero.
Tie all switches to Vref; the output should approach 2×Vref. [Elektroda, Earl Albin, post #21665225]

What should I provide to get precise resistor numbers from the community?

Post the schematic, op‑amp part number, intended Vref, desired full‑scale output, and your proposed R/Rf values. The thread shows responders won’t size parts without these basics. [Elektroda, Earl Albin, post #21665222]

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