AD7877 Analog Devices, AD7877 Datasheet - Page 27

• Manufacturer Part Number: AD7877
• Description: Touch Screen Controller
• Manufacturer: Analog Devices

Specifications of AD7877

• Resolution (bits): 12bit
• # Chan: 9
• Sample Rate: 125kSPS
• Interface: Ser
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref),Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: CSP

Data Sheet
When the DAC output voltage is zero, it sinks the maximum
current through R1. The feedback current and, therefore, V
are at their maximum. As the DAC output voltage increases, the
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
∆I = current at zero − current at full scale
OUT
R
R2 = R
∆I = 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
OUT
Rev. D | 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 the nearest preferred value of 240 kΩ.
∆I = 25 V/240 kΩ − 20 V/240 kΩ = 21 µA.
R1 = 5 V/21 µA = 238 kΩ.
Use the 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
/∆.
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