AN-695 Q&A

Q 

We have recently purchased an ADN8831 chip with the ADN8831 Eval-z evaluation
board. We are currently trying to set up a TEC with a 10k NTC Thermistor module
following the instructions given on the AN-695 Application Note and we are
having a number of questions:

1) The PID network switches S1-5 appear to have different orientation. Could
you please confirm which are ON/OFF positions for each switch?

2) The Quick Start section on page 4 for suggests that we should measure values
of the R1-3 potentiometers after connecting the thermistor and the TEC on the
board. However, as soon as I connect these I cannot measure the pot values
accurately.

3) In the Configure setpoint temperature range section it is saying that we can
use the default values to lock a TEC with a 10k NTC thermistor at 25 degrees.
In page 5 in the same section it is given another method to set a single point
temperature instead of a range. Could you please explain what is the actual
difference between these two settings, and what is the best way to set
accurately a TEC with a 10k NTC thermistor at 25 degrees or any other
temperature?

4) In the Monitor the TEC current section it is given a formula to calculate
the current flowing through the TEC based on the Vitec voltage. However, for
Vref = 2.5 V when the Vitec drops below 1.25 the calculated current takes a
negative value. What is happening in that case?

5) When we set Vtempset at 1.25 V (correspond to 25 degrees for the 10k
thermistor we use) the Vtempout voltage continuously oscillates around the
setpoint voltage. By tuning the PID parameters we manage to reduce the
oscillation amplitude but not to get a stable output. Any ideas what might be
the problem?

 

A 

1) The orientation has following logic: if switch ON, it adds capacitor as the
capacitors are connected in parallel.
For resistive network, if switch is On, it shorts resistor because the
resistors are connected in series. The snapshot of the board schematic
illustrates this (attached).

2) It appears the instructions in the Quick-Start section are not clear enough.
The potentiometers should be verified BEFORE connecting the thermistor. The
reason is apparent from our schematic:  the thermistor completes a parallel
path between TPR1 and TPR123 that will distort the potentiometer measurement.
We revise the instructions in AN-695 accordingly.

3) The default values on the application board assume a particular thermistor
is being used that at 35C has a value of 6.9k, at 15C has a value of 14.8k and
at 25C has a value of 10k. It attempts to linearize the response over that
range. If a larger range is desired, then the thermistor values at High and Low
would be different and the resulting R1, R2 and R3 will also differ from the
"defaults". IF you select a thermistor, that has a 10k resistance at 25C, then
the default settings should produce a setpoint of 25C, but if Rhigh and Rlow
differ from the example, the range will differ.

When only a narrow temperature range is required, the linear approximation of
Vtempout and Temperature is valid and the potentiometer settings can be R1=RMID
(generally 10k) and R2=0.

Our recommendation is to initially use the default settings and verify if these
permit the required setpoint for setpoints other than 25C. If you find that
over your required temperature range Vtempout varies from 50mV (at the low
temperature) to Vref (at the high temperaure setpoint) the default values are
adequate. If your range for Vtempout is less, then your application can benefit
from adjusting the potentiometers to improve the setpoint resolution. Finally,
if you cannot achieve the required setpoint range, you will need to calculate
the potentiometer settings as appropriate for your thermistor and required
temperature range.

But first try the default settings.

4) The TEC sets in the middle of an "H Bridge". The "zero" current level of the
ITEC voltage is 1.25V. So when the current is zero, the output voltage of the
ITEC pin will be 1.25V (nominally). Likewise, the "zero" voltage output at the
VTEC pin with the TEC voltage is zero is 1.25V. When the TEMPSET voltage is
below 1.25 V, the TEC voltage will be positive (LFB>SFB), so VTEC>1.25V and
ITEC>1.25V. If the TEMPSET is above 1.25 V, the TEC voltage will be negative
(LFB<SFB), so voltage at VTEC pin<1.25V and voltage at ITEC pin <1.25V.

5) This will be very difficult to determine without more information since it
depends on the characteristics of the TEC particular and the thermal
characteristics of the object that is being controlled. It may be possible if
you provide some oscilloscope traces to show what the TEMPOUT and TEMPSET are
doing at the various compensation settings you are attempting.