7
There are other factors to consider such as input bias current (quoted at anything up to 4.5 nA). With 4.5 nA flowing from the input into 100 Mohm, that produces an offset error of anything up to 0.45 volts. That blows the effect of the input resistance out of the water.
But, realistically, with a simulation, it all depends on what is set up in the model ...
2
The LM324/LM358 can only sink a few tens of uA down to (near) the negative rail. Try increasing your resistors and maybe add a pull-down resistor. From the datasheet:
Note that the 50uA is nominal, only 12uA is guaranteed, and that only at 25°C.
I would suspect your SPICE model is not very accurate if the simulation does not reflect actual behavior ...
2
I had channel 2 on AC coupling. My circuit was actually outputting the right waveform but my scope was lying to me. Facepalm
2
You are using the op-amp on a single-ended supply (positive only) but you are feeding an AC signal in and it is centred around 0 V.
You need to bias the non-inverting input to half-supply (6V) with a pair of resistors and add a DC blocking capacitor to the input.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Modified circuit.
...
1
The 357 is not unity gain stable at any frequency. It requires a gain of at least 5. Simulators are unreliable on this sort of thing.
1
I would avoid shorting input and output of the Opamp if you do not plan to desolder it entirely, or to power it off. The additional positive feedback might lead to the amplifier instability (depending on the signal source impedance), and/or possibly damage the opamp.
The better solution is to have the option to turn the opamp to a unity gain stage if you ...
1
For pretty much any IC design (including Op Amps), the design is only valid for properly biased power supply voltages. After all, an IC is a solid block of semiconductor material, with carefully controlled trace amounts of dopant materials implanted. So unlike a circuit built on a breadboad, where everything is isolated unless explicitly connected, within an ...
1
It is as you suspect. An unused op-amp with inputs improperly connected.
From How to Properly Configure Unused Operational Amplifiers
Degradation Conditions
Connecting the input pins together or to the supply rails can lead to device
damage. If configured like this, the input stage can suffer permanent electrical overstress (EOS) damage, as shown
...
1
Changing R227 is not a good idea because you will alter the rolloff caused by C225. Reducing R225 will give more gain without a noticable change in response because C223 is larger than needed. The low frequency corner is currently 4Hz; dropping R225 by half would double the gain and make the corner 8 Hz. It is also much easier to splice a new 820 ohm ...
1
Figure 1. Close-up reveals a non-inverting amplifier.
The gain of a non-inverting amplifier is given by \$ 1 + \frac {R_f}{R_i} \$. The simplest fix is to raise the value of Rf.
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