AN929 Datasheet, PDF (14/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
STATE VARIABLE OSCILLATOR
The state variable oscillator shown in Figure 20 con-
sists of integrators A₁, A₂ and inverter circuit A₃. Each
integrator provides a phase shift of 90°, while the
inverter adds an additional 180° phase shift. The total
phase shift of the three amplifiers is equal to 360° and
an oscillation is produced when the output of the third
amplifier is fed back to the first amplifier. The addition
of capacitor C₄ helps ensure oscillation start-up. The
dual element RTD represented by R₁ and R₂ is used to
increase the difference in the oscillation frequency from
the minimum to the maximum sensed temperature.
C₁
R₁ = RTD_A
A₁
V_DD/2
C₂
R₂ = RTD_B
A₂
V_DD/2
C₄
R₄
R₃
A₃
V_DD/2
R₈
R₇
A₅
V_DD/2
V_OUT
V_DD
R₅
R₆
V_DD
A₄
C₅
V_DD/2
f_O = ₂--¹--_π--
-----------^R----⁴-----------
R₁R₂C₁C₂
If R₁ = R₂ = R, C₁ = C₂ = C, and R₃ = R₄
then f_O
=
-------¹-------
2πRC
R₁ = R₂ = RTD
R₃ = R₄ = R₅ = R₆ = R₇ = 1 kΩ
R₈ = 1 MΩ
C₁ = C2 = 100 nF
C₄ = 20 pF
C₅ = 1 µF
FIGURE 20:
State Variable RTD Oscillator.
The state variable circuit offers the advantage that a
limit circuit is not required if rail-to-rail input/output
(RRIO) amplifiers are used and the gain of the inverter
stage A₃ is equal to one (i.e., R₃ = R₄). In contrast, most
oscillators require a limit or clamping circuit to prevent
the amplifiers from saturating. Amplifier A₄ is used to
provide the mid-supply reference voltage (V_DD/2)
required for the single-supply voltage circuit. Resistors
R₅ and R₆ form a voltage divider, while capacitor C₅ is
used to provide additional noise filtering. A comparator
circuit A₅ is used to convert the sinewave output to a
square wave digital signal. The comparator functions
as a zero-crossing detector with a switching threshold
that is equal to V_DD/2. Resistor R₈ is used to provide
additional hysteresis (V_HYS) to the comparator.
Design Procedure:
Set R₁ = R₂ = R, C₁ = C₂ = C, R₃ = R₄ and R_O is the
RTD resistance at 0°C.
1. Select a desired nominal oscillation frequency.
2. C = 1 / (2πR_of_o).
where: R_o = RTD resistance @ 0°C.
3. Select an op amp with a GBWP ≥ 100 x f_max
where: f_max = 1 / (2πR_minC) and R_min = RTD
resistance at coldest sensing temperature.
4. Select R₃ = R₄ equal to 1 to 10 times R_o.
5. Select C₄ using the following equations:
f_-3dB = 1 / (2πR₄C₄)
C₄ ≈ 1 / (2πR₄f_-3dB)
where: f_-3dB ≅ op amp’s GBWP
RELAXATION OSCILLATOR
The relaxation oscillator shown in Figure 21 provides a
simple circuit to interface a RTD to a microcontroller.
This circuit requires only a comparator, capacitor and a
few resistors to generate a frequency output that is
proportional to the RTD resistance. The accuracy of
this circuit is limited by the poor tolerance and large
temperature coefficient available with the required,
relatively large, capacitor C₁.
The relaxation oscillator functions as a comparator.
Resistors R₂, R₃ and R₄ form a voltage divider that sets
the hysteresis and voltage trip levels. Resistor R₁ and
capacitor C₁ form the RC time constant that determines
the charge and discharge rate or oscillation frequency.
If V_OUT equals V_DD, C₁ charges from the comparator’s
low threshold (V_TL) to the high threshold (V_TH), causing
V_OUT to toggle to V_SS. If V_OUT equals V_SS, C₁
discharges from V_TH to V_TL and V_OUT switches to V_DD.
The voltage-switching process then repeats, which
generates the oscillation.
The accuracy of the relaxation oscillator can be
improved by using a comparator rather than an op amp
for the amplifier. A comparator offers several advan-
tages over an op amp in a non-linear switching circuit,
such as a square wave oscillator. An op amp is
intended to operate as a linear amplifier, while the com-
parator is designed to function as a fast switch. The
switching specifications (such as propagation delay
and rise/fall time) of a comparator are typically much
better than an op amp’s specifications. Also, the
switching characteristics of an op amp typically consist
of only a slew rate specification. The accuracy of the
relaxation oscillator can be improved by using a higher
DS00929A-page 14
 2004 Microchip Technology Inc.