Tool/software:
Dear Team,
the input impedance (ZID and ZIC) is displayed in OPA197. Can these parameters also be specified separately for the LMP7701?
A "Equivalent Internal ESD Circuit" is also displayed in OPA197, also available vor LMP7701?
Kind regards
Bernd
Hi Bernd,
. Can these parameters also be specified separately for the LMP7701?
I do not have the data at the moment. The intial datasheet is published back in 2005 and it has a different standard vs. now days. However, the LMP97701 is very high input impedance op amp (input impedance is higher than OPA197 because of the Ib current).
I checked the Spice model, but I do not see anything obvious. If I am able to figure any measured figure, I will append to the post. This part is similar to LMP7704, LMP7721, which is another excellent high input impedance op amp - can be used to interface with electrochemical cell or electrodes.
A "Equivalent Internal ESD Circuit" is also displayed in OPA197, also available vor LMP7701?
Unfortunately, I am unable to find the equivalent block diagram. I am able to find ESD topology for LMP7721, which it should be similar in LMP7701 as well.
If you have other questions, please let me know.
Best,
Raymond
Bernard,
The LMP7721 is part of the 17MHz LMP7711 family.
The LMP7721 is specifically designed to be used with ultra high impedance sources - like electrochemical cells or pH sensors.
The difference is that on the LMP7721, the "ESD" diodes to the supplies are "bootstraped".
Each ESD diode is actually two ESD diodes in series, with the center point between the two diodes driven to the same potential as the input. This way there is 0V across the ESD diode closest to the input - and thus almost no leakage.
The input impedance is in the Tohms.
The common way to get the equivalent input "impedance", is to take the change of the input bias current across the change in VCM.
Zin = dVCM/dIB
So the bias current changes 3fA across 3V
So
Zin = 3V/3fA = 1e15 Ohms
There are similar graphs in the LMP7701 datasheet (I know...I took them!).
Lets take the worst case - at high temp:
So at 125°C, the bias current changed 450pA over 10V
Zin = 10V/450pA = 22e9 Ohms
So you can use the other graphs to calculate the equivalent input impedance across your expected operating range and temperature.
Hi Bernd,
The only reference I can find to the input capacitance is in this section of the data sheet:
25 pF total. As to how much of it is differential vs. common mode, I can't find this information. But, the OPA192 only has 8 pF total of input capacitance so if you are looking to minimize it, then OPA192 is a better choice.
Note that the best low IB device we have is OPA928; this is recently released, it uses OPA192 as a core device but also uses bootstrapping techniques for low IB.
Regards,
Mike
