Hi ALL
I used INA849 for the input circuit of the IR sensor.
It looks like a normal differential amplifier.
However, it becomes somewhat unstable at startup.
Initially, IN- was connected directly to GND, but as a result of consideration, we added 150KΩ.
In the simulation, AMP is working without any problems.
Please advise if this circuit is inappropriate.
Thank you.
Hiroyuki Takada
Hello Hiroyuki-san,
I ran both DC and AC simulations using TINA-TI and circuit appears to be biased and operating properly.
Adding the 150kΩ resistor to IN- is appropriate, as this causes the input terminals to be impedance-matched at DC which results in a lower effective offset voltage from the input bias current.
As for the start-up issue, I see that you have the same -5V "Vee" voltage from you power supply biasing the npn transistors at your "pyro" node. As the Vee voltage is ramped at startup, the changing voltage at "pyro" will pass through the AC coupling capacitor and present as a differential voltage across the amplifier inputs. You may confirm this by monitoring the voltage at the "pyro" node and at the IN+ node during start-up.
This will put the amplifier in a non-linear state as the maximum linear input voltage is exceeded. Once the Vee voltage is stable at -5V and the voltage at the input pin settles to the DC value, I expect the linear operation of the amplifier will be restored.
I believe this should not cause damage to the device, as the input signal is being ramped along with the supply voltage. However, I recommend verifying this by monitoring the input pins for any abs max violations during start-up.
Regards,
Zach
Hello Zach
Your advice is very understandable and meaningful.
Is it possible that if you exceed Abs Max, it will latch up even if it doesn't break?
Regards,
Hiroyuki
Hiroyuki-san,
When the absolute maximums are violated the device can be damaged and/or the functional operation can be permanently altered. We do not typically characterize all of the failure modes that can occur when the stress ratings are violated.
See footnote (1) of the absolute maximum ratings table which states,
"Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability"
Were you able to measure the input voltage during start-up? Can you confirm that the absolute maximum differential input voltage is being violated?
One solution is to increase the pole frequency of your AC coupling filter by greatly reducing the 47μF capacitor. With the values selected, the high-pass cutoff frequency is 22mHz which allows the ramping power supply voltage to pass through to the inputs. You may adjust the cutoff frequency to ensure the power-supply ramping voltage is blocked while the minimum frequency of interest is passed. What is the minimum frequency of interest? For an IR sensor I would not expect the passband will need to be lower than 10s of kHz range.
You should also consider the thermal noise generated by the 150kΩ resistors. The voltage noise of the INA849 is excellent at 1nV/sqrt(Hz). The voltage noise of a 150kΩ resistor is ~49nV/sqrt(Hz) which will dominate the noise performance of the circuit.
It seems that this high resistance was chosen to reduce the voltage drop due to the 10kΩ output impedance of the IR sensor while giving the INA849 input pins a DC path to ground. This makes sense, however it is worth considering to ensure that the resulting noise is tolerable for your application.
Regards,
Zach
Hello Zach
I measured the input and output waveforms of the amplifier.
As you can see, the output is saturated.
The amp will not respond after this.
This phenomenon was independent of the presence or absence of the sensor.
I also get the same result even if I disable the -5V bias current circuit.
Is this an issue when the power is turned on?
Regards,
Hiroyuki
Hello Zach
I almost understand the cause.
+5V startup was 400ms slower than -5V.
Moreover, until the -5V started up, about 1V was observed at the +IN terminal.
When I changed the power supply circuit constants and started +/-5V at the same time, the amplifier worked normally.
Is there a mistake in my understanding?
Regards,
Hiroyuki
Hi Hiroyuki,
Moreover, until the -5V started up, about 1V was observed at the +IN terminal.
If 1V is observed on the +IN pin before the power supply is ramped, I believe you are violating the abs max ratings of the device. See that the abs max for the signal input pins is 500mV above the positive supply voltage. You may also be close to violating the signal input differential voltage if the other pin is at 0V.
I also see that your +5V supply is going negative before it ramps up to +5V. This increases the magnitude of the abs max violation and can lead to all sorts of unexpected circuit conditions such as what you are seeing.
You may want to include Zener diodes at the power supply pins to ensure the supplies cannot be reversed. Additional Schottky diode at the +IN pin will clamp the input pin voltage as well.
INA849_IR_sensor_Diode_Protected.TSC
This article provides a detailed analysis of this diode protection scheme: https://www.ednasia.com/provide-robust-input-overvoltage-protection-for-amplifier-analog-input-modules/
Regards,
Zach
