Max input current (INA129)

Idan,

The input current of the INA129 is limited by the input protection circuitry to safe values if the input voltage is limited to +/-40V or less. This behavior is conveyed in the graph on the middle-right of page 6.

Can you provide a diagram or describe the condition that you are trying to protect?

Regards, Bruce

Hi,

First, thank you for your quick response!

I need to protect against +-250V serge. I'm using ~10V zener diode so I don’t expect problem from the voltage. However, I'm using 100k ohm input resistor (before the zener), so incase of 250V, I will get ~3mA input zener. Is it good? I'm not sure I know how read the "INPUTOVER−VOLTAGEV/ICHARACTERISTICS" graph…  

I'm working with gain=1, Power Supply: 12V to GND. I put diode on the GND (for reverse voltage).  

Thanks,

Idan

Idan,

If you use a zener diode to protect in this way, the only concern is the current capability of the zener. 3mA is very safe--producing a dissipation of only 30mW in the zener. I suggest that you reduce the input resistor to 10k to avoid error due to input bias current. This would cause approximately a 30mA current in the zener--still a safe level.

Regards, Bruce.

3225.INA129.doc

Hi,

I need this high impedance for not affecting the rest of the circuit…

What exactly do you mean by "avoid error due to input bias current"? The min Input voltage is 5V witch gives ~50uA min that is bigger then the input bias (~10nA). Is there any chance that this won't work?   

In addition, could you tell me what this graph mean (attached herewith)?

Thnaks, A lot!

Idan

Idan,

The amplifier's input bias current flows through your input resistor creating additional offset voltage and noise. This would add an additional 1mV input offset, (10nA)(100k).

Your input voltage of 5V does not react with the 100k resistor to create 50uA input current. Your series resistor is not the same as an input resistance. It is merely in series with the very high input resistance of the INA129.

For additional insight on input bias current and input impedance, you may want to read this:  “I Need High Input Impedance!”

The graph shows the input current limiting behavior of the input protection circuitry when the inputs are overloaded. With your zener diode protection circuit, this circuitry will never become activated. It has no significance in your circuit..

Regards, Bruce.

Hi,

Just before I panic… ( :) ), what do you mean by: "Your input voltage of 5V does not react with the 100k resistor to create 50uA input current". This input resistor is indeed very small regardless to the INA129 input resistor, so how it became so effective? If I'll replace the 100kohm resistor with 10kohm input resistor, is it going to work? In that case, what is the max resistor allow?

Idan

Idan,

The purpose of the 10k input resistor is to limit the current in your 10V zener diode during a 250V surge. The 10k resistor will have minimal effect on the operation of the INA129. It could create an additional 100uV of input offset but this is probably small enough to be insignificant.

As a general rule, it is good to keep a source resistance low enough so that the input bias current times the source resistance is in the same range as the offset voltage, or less. You don't want the offset due input bias current to be a major contributor to offset voltage. There may be exceptions to this guideline but this is a good design goal.

Regards, Bruce.