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How High a Source Impedance can be to Opamp Input?

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Mechatrommer:
is it wise to do thing like in the picture? a 1Mohm and 1Kohm divider to "1Mohm input" type opamp? i believe if R2 is changed to near opamp input impedance, things are going to be nasty? how to specify the source (divider's) impedance seen by the opamp? is it 1Kohm or 1Mohm? noobs :P

ejeffrey:
Yeah, that is fine.  First off, the opamp input sees the two resistors in parallel, so the opamp sees 1 kohm input impedance.  If you increased R2 to near 1 megaohm, you would want to use a FET input opamp.

free_electron:
what do you mean with '1 megaohm type input ' opamp... that is not how opamps are specified.
There is a parameter called 'input bias current'. that is what you are interested in. both the value AND the sign !!! some opamps give current , some opamps take current !
lets say you cave an opamp with a 1 uA input bias current. a current flowing IN the opamp.

you apply 1 volt to your input. you have 1 megaohm. the total current in the system is now limited to 1 uampere.. but the opamp will take that current.... So now you have a problem. you actually have a very low impedance in paralle with the 1K. your voltage divider is completely out of whack ... you were expecting (a ratio of roughly 1/1000 ) and you end up up with something completely arbitray because of current flowing in the opamp.

Let's say the bias current comes OUT of the opamp.... the situation worsens. you have 1 microamp coming from the 1 meg resistor + 1 microamp from the opamp. gives you 2 microamp through the 1 K.. you now measure DOUBLE what you expect !

It gets worse .... the other input of the opamp ALSO draws or delivers current. so you are actually creating a voltage differential between the two inputs.. this delta will get amplified by whatever gain you have.

in precise systems you will see that the designer keeps the perceived impedance for both inputs identical ( by putting a resistor in series with the inputs )

Lesson to be learned : if you are dealing with very high impedance circuitry around an opamp : check the bias current specification for your chosen opamp. A bipolar opamp ( like and LM741 LM324 etc ) has a large input bias current.
JFET opamps also have a bias current ( TL072 TL082 )
Only CMOS input opamps have extremely low input bias current. an LMC6603 goes down to half a femtoampere .. )

So : for high impedance work : use cmos input opamps.

ejeffrey:
I was rather assuming that he wasn't using a 1000:1 voltage divider for a signal of 1 volt.  Even so, 1 microamp is a huge bias current.  10-100 nanoamps is more common for bipolar input opamps.  In any case, a jfet or mosfet amplifier would be a poor choice here as offset voltage is likely to be a bigger problem than bias current due to the low impedance _as seen by the amplifier_ of 1000 ohms.  If the DC accuracy really needs to be in the microvolt range, the only solution is a chopper stabilized amplifier which have embarrassingly low offset voltage and bias current.

One more tip: some opamps have a specified "input offset current" which is less than the input bias current.  This means that both inputs have approximately the same current flow.  If your op-amp is like this you can improve matters by putting a resistor between the output and the (-) input instead of the wire link equal to the parallel impedance R1//R2.  Both inputs will then see the same impedance, and the voltage offsets caused by the bias current will partially cancel.  Make sure to check the data sheet, if the offset current isn't markedly less than the bias current, don't bother, and again this is more a thing you do with higher impedances than 1kohm.  It won't help fix the offset voltage.

Mechatrommer:
the planned opamp is THS3092:
http://www.datasheetcatalog.com/datasheets_pdf/T/H/S/3/THS3092.shtml

bias current: 20uA (±20uA/°C)
offset current: 15uA (±20uA/°C)
offset voltage: 3mV (±10uV/°C)
gain to be used: 10X

now how am i suppose to calculate that? i can dig again the formula in book, but what about the drift? how do i ensure my circuit/signal jump up and down at reasonable percentage accuracy? afaik, i havent read this kind of calculation, and the temperature swing can be very broad.

and the reason for the selected opamp is because its the only i can find with ±15V supply BW up to 100MHz. yes the drift/offset/noise spec is not that interesting, but not much selection from TI's parametric. the reason for the 1/1000X divider so that the opamp can take sort of mains voltage stuff as its input, and why it has to be 1Mohm there, so it has sort of high impedance input. think like a fet probe here that can measure mains. i know the buzz of OPA656, but its expensive, low power supply ±6V iirc. and i'm not sure it can swing for my 10X gain. and i'm using 2 opamp for this setup. 2xOPA656 is a fortune... dilemma :(

about the offset voltage, cant i just offset/bias the opamp output so it can null again? and the problem is... if its possible, i'm trying if the 1/1000X divider can take the 1volt signal :P, might change the spec later? meaning i have to reduce the divider by increasing R2, might also worsen the situation... dillema :P i've been "merry go rounding" for weeks.