Other
R2R resistor ladder
With the R2R resistor ladder you can convert bits into analog values with only using simple resistors.
Formula. V = voltage of digital output, n = bits, only use HIGH bits in formula.
Vout = (1Va + 2Vb + 4Vc + 8Vd ) / 2^n
- Notice the the R and 2 * R resistors. Hence the name "R2R".
- Please play with the four different logic inputs, and try different H and L combinations. See table below for Vout when a Va,Vb,Vc,Vd = 5V.
- Notice changing Vd has more impact on Vout than Va.
- Elongate the ladder for higher resolution (n=bits). Keep in mind the relation of R's and 2R's.
| Vd | Vc | Vb | Va | Vout |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0V |
| 0 | 0 | 0 | 1 | 312.5mV |
| 0 | 0 | 1 | 0 | 625mV |
| 0 | 0 | 1 | 1 | 937.5mV |
| 0 | 1 | 0 | 0 | 1.25V |
| 0 | 1 | 0 | 1 | 1.563V |
| 0 | 1 | 1 | 0 | 1.875V |
| 0 | 1 | 1 | 1 | 2.188V |
| 1 | 0 | 0 | 0 | 2.5V |
| 1 | 0 | 0 | 1 | 2.813V |
| 1 | 0 | 1 | 0 | 3.125V |
| 1 | 0 | 1 | 1 | 3.438V |
| 1 | 1 | 0 | 0 | 3.75V |
| 1 | 1 | 0 | 1 | 4.063V |
| 1 | 1 | 1 | 0 | 4.375V |
| 1 | 1 | 1 | 1 | 4.688V |
TIP
Higher bits have more impact on Vout, thus they require more precise resistors.
Passive Wheatstone bridge
A Wheatstone bridge consists of two parallel voltage dividers. In this building block you look at the difference between those two voltage dividers instead. The output of the Wheatstone bridge is zero (balanced) if the output voltages of the two dividers are the same. With the Wheatstone bridge you can measure small changes in one of the resistor values for instance.
- Notice the voltmeter between the voltage dividers. In this case, the output of the bridge is zero.
- Please play with one of the resistor values, and see the voltage measurement changing.
TIP
Use a Wheatstone bridge together with a differential amplifier to enlarge small differences. See Amplifiers.