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]
