EVAL-AD7877EBZ Analog Devices Inc, EVAL-AD7877EBZ Datasheet - Page 27

• Manufacturer Part Number: EVAL-AD7877EBZ
• Description: BOARD EVALUATION FOR AD7877
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7877EBZ

• Main Purpose: Interface, Touch Screen Controller
• Embedded: No
• Utilized Ic / Part: AD7877
• Primary Attributes: 4-Wire Resistive Touch Screen Controller, SPI Interface, On-Chip: Temp Sensor, Voltage Reference, 8-Bit DAC
• Secondary Attributes: USB GUI, LCD Noise Reduction Feature, 2.7 ~ 5.25 V, Wake Up on Touch Feature
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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