June
2005, Issue 179
Accurate
Capacitance Meter
Cypress
PSoC High Integration Challenge 2004 Contest Winner
by
Alexander Popov, Jordan Popov, and Peter Popov
CURRENT
SOURCE
The
current source is implemented with the differential
amplifier (built with an SCBLOCK). The current is set
through the CurSet DAC and RSET resistor.
The DAC provides the voltage (VSET) relative to AGND
(see Figure 4a, p. 45).
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(Click
here to enlarge)
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Figure
4—The DAC value and RSET are used to set the current
(I) in forumla a. Formula b shows how RefHIGH and
RefLOW set the range between VHIGH and VLOW. Formula
c sets a limit to VOUT so the current source operates
linearly. Formula d is obtained after substituting
all of the above in Equation 1. Rearranging, you
arrive at e, where adjustable and measured parameters
are on the left and system parameters are on the
right. Use f to internally calibrate VREF by using
ADCINT12. |
The
InpBuf unity gain PGA feeds P0[5] to the DiffAmp instead
of port 2, leaving port 2 free to drive the LCD. The
DiffAmp’s output is fed through an output buffer to
P0[4].
To
implement a bidirectional current source, you must use
AGND as the current return path. It’s fed outside the
chip through the unity gain PGA BufAmp to pin P0[2].
