Op Amps Maintain High Input Impedance When Powered Down

Hello Alessio,

Can you please attach the file dn533f.pdf? The only one I can find online doesn't have any op amps in it. Instead it is a design note about a data backup power solution.

Also, it would be very helpful if you could provide your schematic, op amp part number, and the voltages that are being applied to the device and 3Mohm resistor.

Finally, I'm particularly interested in the impedance as seen by the power supply pin when there's "no power supply". For example, is it high-impedance (e.g. open switch), grounded, connected to the output of a power device, or in some other configuration?

Sure,

the doc is regarding the LT6015/16/17 that should maintain the high input impedance when the op-amp is powered down.

Anyway this kind of op-amp has still a low input impedance for my application since that I need at least 100MOhm at the + input.

My schematic is simple, just imagine to have a resistor partition where one of these change the value in function to a fisic parameter.

This partition goes into the + input of the op-amp, while the - is to a fixed voltage value around VCC/2.

The op-amp could be powered off in order to save power but the "sensor" and some other components around are still powered by a battery having 1uA as maximum consumption.

The issue regarding the change in the input impedance of an op-amp when powered down seems to come from the internal diode for the ESD.

I tried, in order to maintain the high input impedance at inputs, to put a series diode on the op-amp power supply with no success (my case expects a 3.3V single supply).

Currently the only solution seems to disconnect the op-amp inputs with a couple of N-MOS in series that cut off the current path.

I would be better to have an op-amp that still have am input resistance of, at least, 100MOhm even when powered down.

That's why I've opened this thread.

Thanks

dn533f.pdf

Alessio,

Note that "VCC = positive supply" in this post

There is no need to get input range above VCC to get high input impedance for power down. There are many op amps that have this feature however, there is no good way to search for them.

Start by finding op amps you like then look for input range in AMR table.

Here is TLV342 AMR table. See that input is limited by max voltage and not listed as related to VCC, for example "VCC + 0.3V"

Even if input is not listed as limited by VCC, be sure to ask on e2e or test samples yourself. 

This table example gives more confidence because output range did mention Vcc and input does not.

AbsMagEnc.pdf

Hi Ronald,

my question was regarding how to maintain the high input impedance even when the op-amp is powered down.

I don't want to use the TLV342S because in shutdown can consume up to 1.5uA (as per datasheet).

Rather I can use an ultra low power op-amp like the TLV522 that has the same power consumption when powered.

BUT I NEED TO REDUCE THE POWER CONSUMPTION AS MUCH AS POSSIBLE IN ORDER TO PRESERVE THE BATTERY.

Attached you can see my actual configuration in order to cut off the Sin and Cos input from the +/- op-amp input when there's no 3.3V available.

In that case everything is powered by a battery.

So in my case will be better to use an op-amp which can let me to eliminate the N-MOS if I can maintain the high impedance feature.

Alessio,

I checked the data sheet for low current duals (in order) LPV802, LPV812, TLV8542, TLV8802, TLV8812, TLV522, LPV542, TLV2402 OPA2369, LMC6442, TLV2369, and OPA2349. You can check higher ICC devices yourself if you like, but suspect over 30 will be need be checked to find one. Do you have a maximum ICC in mind?

Of that list, only TLV2402 looks promising. However, I do not know the input current when powered down. Start a new e2e post with that part number as it is not my device and I have no samples in house to test. 

How much current is allowed to be drawn from or injected to the input signal?
If zero I can't help. If some small finite value, I'll try to come up with a buffer circuit. 

I'm using a low Icc type (TLV522) because previously (revision 0 of the board) the analog section was supplied all the time. So I chose that promising op-amp.

Now it's not an issue anymore since that I cut off the power for the analog section.

So feel you free to choose an op-amp that must be input and output rail-to-rail, single power suppling (from 2.5V), dual in MSOP8.

The input current must be the lowest possible with a input resistance at least of 50MOhm even when powered off.

Thanks