AD7877 Datasheet, PDF (27/44 Pages) Analog Devices – Touch Screen Controller

VOUT falls. If the DAC output exceeds VREF, it starts to source
current, and VOUT has to further decrease to compensate. When
the DAC output is at full scale, VOUT is at its minimum.
Note that the effect of the DAC on VOUT is opposite in voltage
mode to that in current mode. In current mode, increasing DAC
code increases the sink current, so VOUT increases with
increasing DAC code. In voltage mode, increasing DAC code
increases the DAC output voltage, reducing the sink current.
Calculate the resistor values as follows:
1. Decide on the feedback current as before.
2. Calculate the parallel combination of R1 and R3 when the
DAC output is zero:
RP = VREF/IFB
3. Calculate R2 as before, but use RP and VOUTMAX:
R2 = RP(VOUT(MAX) − VREF)/VREF
4. Calculate the change in feedback current between
minimum and maximum output voltages as before using
∆I = VR2(MAX)/R2 − VR2(MIN)/R2
This is equal to the change in current through R1 between
zero output and full scale, which is also given by
∆I = current at zero − current at full scale
= V/R1 − (VREF − V)/R1
= V/R1
AD7877
5. R1 = VFS/∆.
6. Calculate R3 from R1 and R using
R3 = (R1 × RP)/(R1 − RP)
Example:
1. VCC = 5 V and VFS = VCC. VOUT(MIN) is 20 V and VOUT(MAX) is
25 V. VREF is 1.25 V. Allow 100 µA around the feedback
loop.
2. RP = 1.25 V/100 µA = 12.5 kΩ.
3. R2 = 12.5 kΩ × (25 Ω − 1.25 Ω)/1.25 Ω = 237 kΩ.
Use nearest preferred value of 240 kΩ.
4. ∆I = 25 V/240 kΩ − 20 V/240 kΩ = 21 µA.
5. R1 = 5 V/21 µA = 238 kΩ.
Use nearest preferred value of 250 kΩ.
6. R3 = (180 kΩ × 12.5 kΩ)/(180 kΩ − 12.5 kΩ) =13.4 kΩ.
Use nearest preferred value of 13 kΩ.
The actual adjustment range using these values is 21 V to 26 V.
Rev. A | Page 27 of 44