AD8495: How to set up the setpoint controller

Document created by analog-archivist Employee on Feb 23, 2016
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I am trying to use AD8495 in the setpoint controller  with hysteresis (very
similar to Figure 36 in the official spec) but I’m having a trouble –
controller is latching in low output state after certain time of normal
operation. Does AD8495 have latch mode or this testifies damaged component?
I would also add, that this is a single application for measurement/lab purpose
only. Will be not produced in series.

 

There are only a few things that could cause an issue using the AD8495
(http://www.analog.com/AD8495_datasheet). In the following I have a few
debugging steps to help understand why you are not seeing the expected result.

First I would recommend to simplify it first and configure the AD8495 as in
figure 35 of the datasheet on page 15. This is done by leaving the output pin
open for no hysteresis and tie the reference pin to an appropriate voltage or
ground. For your reference I’ve attached the circuit below.

In fact it would be even simpler, if you have a thermocouple connected, to
disconnect this and short the inputs to ground so the AD8495 itself acts as a
temperature sensor.

Using for example a setpoint voltage of 250mV (correlates to 50°C) at room
temperature (~25°C) the output will rail to the negative supply voltage. What
supply voltages are you using in your setup? Are you on a single supply?
For testing purposes I would recommend to vary the setpoint voltage and see if
the output rails positive once you go below ~125mV. This should tell if the
setpoint action is working.

If the setup is working with the AD8495 inputs shorted and setup as a setpoint
controller with no hysteresis, I would recommend as a second step to add the
feedback resistors for the hysteresis and verify if it’s still working as
expected.
The last step would be to add the thermocouple to the inputs. Please note that
we strongly suggest to ground on side through a 1MOhm resistor as shown as in
figure 29 on page 13 of the datasheet. For your reference I’ve attached the
relevant section below.

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