PGA280: Input switch leakage

Hi Ben,

Benjamin Bawkon said:

Is there any data or specifications for the off-state leakage current of the input switches (A1/A2 & B1/B2)?

I am unable find the data in our database. I probably have to ask for the designer. How close do you need to know about the input impedance or leakage current in the mux block? 

On most multiplexers, the input capacitance is close to about the same when a switch is closed or open, or at least in the same order of magnitude.  Therefore, I ‘suspect’ the input capacitance with the switch open is likely also around the low ~10s of picofarad range.  Since the PGA28x offers very high impedance >1GΩ and low input bias current in the ±2nA range, I tend to think, the switch in open position would also exhibit low leakage.

Best,

Raymond

Hi Raymond,

I'm less concerned with input capacitance than I am with leakage current, as long as capacitance isn't absurdly high - which I don't expect it is.

I'm dealing with some very low-level signals with high input impedances, so I'm down in the weeds counting picoamps.  I've got very low Ib buffers in front of the PGA280 for just this reason.  A couple of nanoamps would be quite detrimental to my signals.  I'm hoping that off-state leakage on the switches is a small portion of the +/-2nA input bias.

If you could get any information from the designer, that would be really helpful.  Again, I'm quite interested in the full temperature range.

If I have to, I'll put a low-leakage analog switch in series with the PGA280s switches to reduce the off-state leakage, but I hate throwing parts at a design without knowing if they're really necessary.

Thank you for your help,
Ben

Hi Ben,

Please give us a couple days and I will see if I am able to get the information for you. 

Best,

Raymond

Thank you Raymond!

Hi Ben,

I contacted the PGA280's designer and we still do not have the data to answer your questions directly.  

Per PGA280's Mux input block, ESD diodes are before the mux switches and there are always a few on/off switches connected to the input, I think it would be quite safe to assume that the leakage mux off impedance would be quite a bit better than 1 GΩ. 

Since your application is looking for <10s pA in leakage current at mux off position, PGA280's input impedance is unable to meet the ultra low leakage current requirements. The leakage currents via the ESD diodes in front of mux switches will likely determine the magnitude of leakage current level at 25C and over temperature. PGA280 is unlikely to meet <10s pA in leakage current per your application (unless the PGA280 is fabricated with ultra low leakage ESD diodes, which is unlikely for the mux switches). PGA280 is designed from a IC process prior to 2009. 

For instance, one of our high performance OPA192 precision op amp, its input bias current is in an order of ±5pA (25C) to ±20pA range (over temperature) and this is op amp's input. The input bias current is likely determined by the ESD diodes of the analog front end at the input. 

Please take a look at OPA3S328, and see if it will meet your switching requirements. OPA3S328has ultra low input leakage current. The op amp has 3 internal gain switches, and the the leakage current of these switches are still in an order 5-10pA at 25C. It is possible that it may meet your application (my guess). 

If you have additional questions, please let us know. 

Best,

Raymond