Hi,Hi,
a remark on the simulation. First: well done!
you see that I_CC (= current on VCC) carries AC while I_EE does not.
If you shift V_IN (common mode as well as differential mode) you may see the opposite (I_EE carries the AC) or both carry AC.
Klaus
example:Hi,
How to shift that? i did not get it
Can you guide how to that?
What exactly does it confirm?SIM confirms this :
This OA has 130 MHz bandwidth , much more than an INA.Use an INA ... and don´t care.
True.This OA has 130 MHz bandwidth , much more than an INA.
* that AIN+ has not the same input impedance as AIN-.
*
This is not what I´m talking about:When used as a balanced Diff Amp the Differential Rin will be 2x Rin (=4k81) since the differential input is a virtual null from negative feedback.
The Rin of AD7484 (Vin) > 10 Meg // 35 pF with 1uA bias current.
The sim show's the current thru the input R1 and R3. Shows it AC wise to be the same.What exactly does it confirm?
I don´t understand what the SIM actually wants to show/measure.
*****
In my statement input impedance of AIN+ is not the same of AIN-
...
Don´t you need individaul measurements for AIN+ and AIN- to confirm?
Let´s take the setup of your SIM:
1)
Connect AIN+ to GND, connect AIN- to GND.
Measure the currents at each input individually.
Both should show close to zero. Equal so far. But without current and without voltage one can not calculate resistance.
2)
now leave AIN- at GND.
apply different voltages at AIN+ and measure the current at AIN+.
1V --> 166.4uA according Ohm´s law: R = V/I = 1V/166.4uA = about 6k (V_out = 0.249V)
2V --> 332.8uA = about 6k (V_out = 0.499V)
5V--> 832uA = about 6k (V_out = 1.247V)
-5V --> -832uA = about 6k (V_out = -1.247V)
3) now simply swap channels
leave AIN+ at GND
apply different voltages at AIN- and measure the current at AIN-.
1V --> 207.9uA according Ohm´s law: R = V/I = 1V/166.4uA = about 4.8k (V_out = -0.249V)
2V --> 415.8uA = about 4.8k (V_out = -0.499V)
5V--> 1040uA = about 4.8k (V_out = -1.247V)
-5V --> --1040uA = about 4.8k (V_out = 1.247V)
My conclusion: The input impedance of AIN+ is about 6k, while the input impedance of AIN- is about 4.8k. --> not equal.
You can do the same with AC and get the same results: 6k vs 4.8k
Now you can say: But the output voltage is correct in both cases. Why care about input impedance then?
Fair enough!...As long as the source impedance is ideal zero Ohms, there is no problem. (have you ever seen anything "ideal"? ;-) )
Let´s say you have 1V at AIN+ and 1V at AIN---> difference = 0 ... thus you expect 0.000V at the output.
Now install a 1k as source impedance at AIN+ --> Vout = -0.036V
But when the 1k is in the AIN- path then --> Vout = +0.034V (the absolute value differs by about 2mV to the above result)
It isn´t even symmetric!
To avoid these known errors .. I recommended to use a true instrumentation amplifer that does not suffer form these problems.
This is my recommendation for "hobbyists", because it is simple and accurate. Use an INA ... and don´t care.
For sure an experienced electronics designer may say "I know what I´m doing, I know the flaws and errors of the circuit, I know how to calculte them, I know how to handle them ..".
Well, nothing against it. Maybe it saves a few cent per device.
Klaus
This is not what I´m talking about:
I´m referring to AIN+ and AIN- of schematic of post#3.
What I say: R_AIN+ is not the same as R_AIN-.
See my example:
Set AIN+=0 and apply a signal on AIN-. Measure voltage and current of AIN- --> calculate R_AIN-
Set AIN-=0 and apply a signal on AIN+. Measure voltage and current of AIN+ --> calculate R_AIN+
They are not the same! One is 6k the other is 4.8k.
I´m looking for an application note.
Klaus
I did not change the part value for the measurements above.Of course if you change the input values the Z changes, but the point was counter to your
earlier post.
Prove it by gounding one input and applying 1V to the other input and mesure the input current.Post #3 is a balanced differential input for both stages. Not unbalanced. Input currents are matched within Iio limits.
Again: this is not what I´m taking about.e.g. Note that I have injected +/-5V of common mode 60 Hz on the balanced impedance.
The AIN's correspond to my sim R1, R3 inputs.Again: this is not what I´m taking about.
Please read my posts! I didn´t alter by adding "1k" for the impedance measurements!If you alter the schematic as show in POST #3 by adding your 1K of course the circuit
behaves differently.
No!We are discussing the same input impedance property, single or differential. Note the 50% reduction is matched.
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