+ User Manual
Protection
The pair of Schottky diodes (D2) in combination with the 470 Ohm resistor protect the ADC
input against over-voltage caused by ESD or shorts to the supply.
Scaling
The PWR_THRM0 reference voltage is feeding a voltage divider that has the thermistor in the
lower section and a 2k05 resistor in the upper part. The thermistor is wired parallel with the
voltage divider that feeds the ADC inputs. The ADC measures half (more exactly 0.4885 times)
the voltage present over the thermistor
TSCADC
As the PWR_THRM0 voltage is twice the reference used by the BeagleBone's ADC, the ADC
returns the voltage divider ratio directly (rather than a voltage).
ADS1015
The ADS1015 converter has programmable gain and differential inputs. Normally this device
would be configured for single ended operation over a 0 - 2.048 Volt range resulting in 1 mV
resolution.
Voltage inputs
Instead of using thermistors on the analogue inputs, it is also possible to measure voltages
directly. Depending on the internal resistance of the source is being measured, it may be
necessary to remove the 2k05 resistors for best results (R22 in illustration 7, R28 and R34 for
the other inputs).
Thermistor curve
The analogue input circuit is optimized for the widely used Epcos B57560G104F glass-bead, or
a similar 100 kΩ thermistor. The curve slope is positioned to give best resolution over the
entire 25 to 250 degrees Celsius operating range.
The thermistor can be connected directly to the two pins of the analogue input connector. It
will withstand up to 12 Volts on the signal side (pin 1) without damage to the BeagleBone.
This may happen during an accidental short-circuit of a thermistor wire to a supply or heater
wire.
† As long as the input voltage is within range and not being clipped by the over-voltage protection.
version 1.4.9 (09/04/14)
page 26