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Original question:

INA826S: output impedance when disabled

INA826S: Shutdown Mode Impedance, definitive answer?

MeDevelopment
Prodigy 30 points
Part Number: INA826S


In these two threads: 

The answers from TI differ although they don't completely constrast with each other. 

I would need to control the same node with a DAC instead of the INA826S, as an alternative operating mode (speed and glitching thus aren't very relevant to me), so I would need to have the INA826S not just shutdown but also disable its output. 

In one post it seems that it's just a shutdown mode and nothing is guaranteed so it would probably not work, while in the other post Marek talks about the INA826S being actually "disabled" and presenting a high impedance as if it's a specification. 

Would you be able to confirm which is the reality? The datasheet doesn't seem to express itself on this matter. 

Thanks for your help. 

  • 0
    Guru 140200 points
    Hi Robert,

    I think the main purpose of the shutdown feature is to reduce the supply current of INA826S, not to support multiplexing applications. And if the datasheet does not specify the output impedance versus applied output voltage during shutdown, then I would not rely on that the output does what you want.

    Of course, shutting down the chip and by this the output stage will cause a high ohmic output impedance. And figure 61 and 62 of datasheet show, that the output voltage goes down to 0V with a load of 10k, indicating that the output does not generate a voltage by itself and presents a high output impedance. But the interesting question is, whether the output stays high ohmic, when you apply a signal voltage to it.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 30 points
    >I think the main purpose of the shutdown feature is to reduce the supply current of INA826S, not to support multiplexing applications. And if the datasheet does not specify the output impedance versus applied output voltage during shutdown, then I would not rely on that the output does what you want.

    that's my thinking as well, but a multiplexer spared means space spared, so I deemed it worth asking given what's written in the second thread.
  • 0 in reply to MeDevelopment
    TI__Guru 68511 points

    Yes, the main purpose of the disable function in INA826S is to lower the current from typical 200uA down to 2uA (max 6uA).  But to answer your question regading the output impedance in a disabled mode, please take a look below at the functional blok diagram of INA826S - I added in red small leakage currents present during shutdown.  The actual magnitude of these leakage currents depend on the value of Rg resistor (thus gain) and supply voltage but under most circumstances should be below 1uA.

    Looking from the output terminal to REF pin, you will see six resistors with total value of 300k plus Rg and a small leakage currents and Vout is effectively floating. But connecting a resistive load (even voltmeter) at the output will tend to move Vout in the direction where the other side of the load is connected.  If you connect a low value resistor between Vout and ground, Vout will virtually collapse on ground but if instead you connect it to either rail, Vout will get close to respective rail.  In case you connect high value resistor between Vout, it will create a voltage divider with ~300k internal input resistance and Vout will only partially move in the direction where the other side of the load is connected.

    In a shutdown mode, output does NOT respond to input signal but instead self-biases based on the internal leakage currents.  However, in case of output being driven by the AC signal, and depending on the frequency of applied AC signal, the output will change its output impedance due to parasitic capacitance of the output transistors - despite the fact that in disable mode the output stage is starved of current, there are still some junction capacitors present that will influance the actual AC output impedance.  Of course, driving the output above or below rails will turn on ESD clamping diodes.  All in all, you should be able to use INA826S in multiplexing application as long as you drive the output with low impedance source and you give enough time between diffferent channels for the disabled channel to actually become high impedance (minimum 10us) before enabling the other channel (min 60us).