AN135 データシートの表示（PDF） - Xicor -> Intersil
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AN135 Datasheet PDF : 5 Pages
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Xicor Application Note
AN135
Photodiode
392 Ω
100K Ω
392 Ω
1MΩ
VO
IS
2
–
1
Gain
Adjust
DCP1
+5V
6–
8
A2
+
3
A1
+5V
+
5
7
4
392 Ω
–5V
Hamamatsu
1226-5BK
µC
Control
Bus
and
Memory
A1= A2 =
1 AD822
2
– 5V
X9258T
1MΩ
DCP2
Zero
Adjust
–5V
+5V
100K Ω
Figure 2. Programmable Transimpedance Amplifier
Of particular interest is the pseudo-logarithmic behavior
of the circuit's transimpedance gain as a function of P1.
The transimpedance gain of this amplifier varies over
the range of 1/256 megohm to 256 megohm as the
programming of DCP1 varies from 0 to 255 while gain-
factor resolution never gets worse than 10% per P1
increment over an 400:1 (52dB or nearly 9 bit) range of
50K to 20MΩ. Transimpedance settings covering an
even wider span are accessible, 4K to 256MΩ
corresponding to fullscale IS values from 1mA to 16nA,
albeit with reduced resolution. A linear gain adjustment
cannot achieve both a multi-decade gain adjustment
range and good adjustment resolution throughout the
range.
A second feature of the circuit in Figure 2 is the
independence of gain of both DCP1 element and wiper
resistances. Using the pot wiper as an input terminal
effectively moves element tempco and wiper contact
resistance errors inside the feedback loop of A1, thus
removing them as gain-error terms and thus improving
the time and temperature stability of the gain setting.
DCP2 is used to null the amplifier zero point. It varies the
voltage at the noninverting input of A2 by ±2mV with a
resolution of 16µV.
PROGRAMMABLE PRESSURE TRANSDUCER
CIRCUIT
The silicon piezoresistive-bridge pressure transducer
(SPPT) is a dominant technology in automotive,
industrial, medical, and environmental pressure sensor
applications. All SPPTs share a similar architecture in
which a thin (5 to 200µm) micromachined silicon
diaphragm incorporates an implanted piezoresistive
Wheatstone-bridge strain-gauge. Applied pressure
bends the diaphragm, imbalances the strain gauge, and
thereby produces a differential output signal
proportional to the product of pressure times bridge
excitation voltage.
SPPTs must be supported by appropriate signal
conditioning and calibration circuits. Finite elasticity
limits the SPPT diaphragm to relatively small deflections
which generate only ±1% modulation of the bridge
resistance elements and low signal output levels,
creating the need for high gain, low-noise, temperature-
stable DC amplification. The signal conditioning circuit
must also include stable, high resolution, preferably
non-interactive, zero and span trims. The automation of
the calibration of the sensor circuit is an enormous
benefit in the production environment.
Another complication of SPPT application is the large
temperature dependence of both total bridge resistance
and peizosensitivity (the ratio of bridge output to
excitation voltage times pressure). Bridge resistance
increases with temperature while peizosensitivity
decreases. Some SPPT designs (e.g. the Lucas NPC-
410 series) carefully equalize these opposite-sign
tempcos. The payoff comes when such SPPTs are
excited with constant current because the increase with
temperature of bridge resistance (and therefore of
bridge excitation voltage) then cancels the simultaneous
decrease of peizosensitivity.
AN135-2
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