Mindray Passport 5-Lead, Service Manual

Page: 221 / 544

Passport 5-Lead, 5L, LT, XG Service Manual 0070-00-0420 2 - 19
Theory of Operation Block Diagrams
The voltage output of U48B is proportional to the resistance placed in the feedback loop of
U48A. By turning on one of the switches Q8, Q9, or Q10 the resistance selected will be
either the probe or one of the resistors, R2 or R3. These are precision resistors used as
standards for accurate determination of the probe resistance. Resistor R2 is used with probe
series 400. Its value, 3539 ohms 0.025% is equal to series 400 probe at 15ÉC (bottom of
measuring range). Similarly, R3 is used with probe series 700. The CPU “knows” which
probe is plugged in (explained later) and selects the appropriate standard resistor. Following
is the measuring sequence for probe series 400. The probe is selected, by signal TA and Q8,
resulting voltage Up4 at the output of U48B. The voltage is digitized by the 12 bit A/D
converter (U21) and stored by the CPU. Then the standard resistor R2 is measured by turning
Q8 off and enabling Q9 with TB, resulting in voltage Us4. The CPU then calculates the probe
resistance: Rp4 = R2 ! Up4/Us4. Assuming that the circuit gain and measuring current are
stable through the brief measuring period, accuracy of the probe resistance determination is
limited by the A/D conversion process. Using a 12 bit converter, accurate to ½ LSB, worst
case error in terms of temperature is 0.12°C.
Series 700 probes have two thermistors, Th1 and Th2. The first one is used to measure the
temperature, in the same way as described for series 400. The second thermistor is used to
determine which probe series is plugged in as follows: Op-amp U47B compares the voltage
on the ring terminal of the input jack with a threshold of 0.7 volts, from R193 and R177.
When a series 400 probe is plugged in, the ring is shorted to the sleeve contact, which in
turn is tied to the output of U48A. With the switches Q7, Q8 and Q9 off and Q10 on, the
ring voltage is about 1.2 volts and the output of U47B is high. When a series 700 probe is
connected, thermistor Th2, connected between the ring and the sleeve, forms a voltage
divider with R152 at the positive input of U47B. With the switches in the same state as
before, the ring voltage is 0.5 volt or lower and the output of the comparator is low. The
probe type information is used to select the correct set of constants for the temperature
calculation, correct standard resistor and for the selection of the probe measuring current.
The current is adjusted to approximately 330 microamps for series 400 and to 125
microamps for series 700, by means of switch Q7, activated by signal TD. This is done for
optimal utilization of the A/D dynamic range.
The probe jack, located off board, is arranged to short the tip and sleeve terminals when the
probe is unplugged. This provides a zero A/D converter reading as an indication of sensor
disconnect. Primary ESD protection is in the form of a zener diode located at the jack.
Additional protection is achieved by the R106 and C92, as well as the 100 ohm damping
resistors on the gates of Q7-Q10. Because the probe cable capacitance is present at the
summing node of U48A, C7 is used, in conjunction with resistors R92 and R120, to provide
local AC feedback to isolate U48A from the capacitance and ensure stability.
Calibration of the circuit is performed by measuring the two precision resistors R2 and R3 for
equivalent temperature determination. R2 replaces the probe and R3 serves as the standard.
Resulting ratio is R2/R3 = 3539/9428 = 0.37537. Nominally this ratio corresponds to
37.48ÉC. Assuming worst case errors, this number will be 37.5, ±0.1°C.