CAV444 Datasheet, PDF (6/12 Pages) Analog Devices – C/V transmitter IC with adjustable output voltage for capacitive input signals

CAV444 – C/V transmitter IC
with adjustable output voltage
for capacitive input signals
Equation (2) implies that signal V_TPAS is amplified by an internal operational amplifier in the output
stage, where gain G_LP can be determined by resistors R₁ and R₂. Resistor R_A is used to set the supply
for the f/V converter. Pin VB is for internal biasing and must be connected up to supply voltage
V_CC.
With equations (3) and (4) incorporated into (2), the transfer function for the full-scale signal (1) is
accrued as:
V_OUT * = V_DIFF
* +V_REF
= G_LP
⋅ V_TPAS
+ V_REF
= _{}1 +
R₁
R₂

⋅

3
⋅
C_M
8⋅
⋅ ∆V_CM ⋅ R_CM
C_W ⋅ R_CW
 ^{+ VREF}
(5)
where
C_W
=
C_{M ,max}
1.6
.
The
resistors are
defined by the
respective
load currents
and
have
fixed
values
of R_CM = 250kΩ and R_CW = 500kΩ.
We can see that the output voltage is a linearly dependent function of measurable variable C_M
(V_OUT* = f(C_M)), as all other variables are fixed by the dimensioning process.
V_{O U T}
V_REF 2,5V
C_M
C_{M Min}
C_{M Max}
C_M
Figure 4: Output signal V_OUT* referenced to ground
In Figure 4 we can recognize that the output signal is raised by V_REF. In order to be able to fix the
offset, the network must be extended.
TRANSFER FUNCTION (WITH ADDITIONAL OFFSET
ADJUSTMENT)
When setting the output signal the adjustability of the offset must be taken into account with the
transfer function (5). With regard to Figure 5, the transfer function is calculated as:
V_OUT = V_DIFF + V_REF = A ⋅V_TPAS + B ⋅V_REF
(6)
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Mai 2010 - Rev 1.1 - Page 6/12