f75385 Feature Integration Technology Inc., f75385 Datasheet - Page 9

• Manufacturer Part Number: f75385
• Description: ?1o C Accuracy Temperature Sensor Ic Datasheet
• Manufacturer: Feature Integration Technology Inc.

6.7 ADC Conversion Sequence
6.8 Thermal Mass and Self Heating
6.9 ADC Noise Filtering
6.10 PCB Layout Guide
Hysteresis values.
after the end of conversion. A BUSY status bit in the status byte shows that the device is actually performing a new
conversion; however, even if the ADC is busy, the results of the previous conversion are always available.
SOP package is about 140 in still air. For the F75385 junction temperature to settle to within +1°C after a sudden
+100°C change requires about five time constants or 12 minutes. The use of smaller packages for remote sensors
such as SOT23, improves the situation. Take care to account for thermal gradients between the heat source and the
sensor package do not interfere with measurement accuracy. Sel-heating does not significantly affect measurement
accuracy. Remote sensor self-heating due to the diode current source is negligible. For the local diode, the worst case
error occurs when auto-converting at the fastest rate and simultaneously sinking maximum current at the ALERT#
output. For instance, at an 64Hz rate and ALERT# sink around 0.7mA when pull up resistor 4.7K ohm to 3.3VCC, the
typical power dissipation is VCC x 220 uA plus 0.4V x 0.7mA. Package θJA is about 120 °C/W, so with VCC = 3.3V
and no copper PC board heat-sinking, the resulting temperature rise is:
high-frequency noise rejection; therefore, careful PCB board layout and suitable external filtering are required for
high-accuracy remote measurement in electronically noisy environment. High frequency EMI is best filtered at D+ and
D- with an external 2200pF capacitor. Too high capacitance may introduce errors due to the rise time of the switched
current source. Nearly all noise sources tested cause the ADC measurement to be higher than the actual temperature,
depending on the frequency and amplitude.
sensor, so care must be taken to minimize noise which is occurred at the sensor inputs. The following guideline
should be taken to reduce the measurement error of the temperature sensors:
If a START command is written, both channels are converted and the results of both measurements are available
Thermal mass effect can seriously degrade effective accuracy of the F75385. The thermal time constant of the
Even with these contrived circumstances, it is difficult to introduce significant self-heating errors.
The ADC is integrating type with inherently good noise rejection. Micro-power operation places constraints on
PCB can be electrically noisy environments, and the F75385 is measuring very small voltage from the remote
dT = 1.01mW x 120 °C/W = 0.12 °C
-6-
July 2006
V0.21P