How to choose the right amplifier?

[berger.h]

I'd like to clarify a few things about the choice of operational amplifiers
I borrowed the scheme published here on the forum
The author used exclusively AD8620 amplifier, unfortunately pricing policy of European distributors (Farnel, Mouser TME) makes it a very expensive.

The question is where would be cheaper to use other amplifiers without appreciable loss parameters

Red stage
It si Instrumentaton amplifier
suppose the requirement for minimum input current
Power supply +-5V - +-8V
maximum amplify 20x
and input frequence DC-100kHz
input voltge level 0 - +-1,25V



Green stahe is 4th LPF on 100kHz
Amplifier have A=1x

Blue Stage,
Inverting ampélifier and level shifter
make from +-1,25V 0-2,5V


Which amplifier AD8620 is possible in this design to replace cheaper, without a significant loss of properties of the whole chain?

**broken link removed**

 

[KlausST]

Hi,

You say DC-accuracy: Bu you have a high pass filter at IN+. This kills DC. Are yo sure you need DC accuracy?
Now DC is only from then IN- input. Does it make sense?

*****
With the given informations there are many OPAMP doing that job.
Look for:
* supply voltage range
* check output voltage range. Maybe you need RR outputs
* inputs don´t need to be RR
* Gain of 20 with 100kHz of signal frequency: Look for OPAMPs with GBW of at least 5MHz. (depending on input signal waveform and allowed distortion)
* for the first two INA opamps: look for very low Ibias. Example: 130pA (typ at 85°C) of bias current cause an input offset voltage of 130mV !!!!
* for the other OPAMPS IBias is not that important.

* for the first two INA opamps:look for low current noise.
* for the other OPAMPs noise will not be the problem, because I assume noise is dominated by the first stage.

Klaus

 

[berger.h]

DC-accuracy, you are right, for simplicity, let's assume the removal of the blocking capacitor.
From the other side, 0V is DC too and input offset voltage on on each amplifer make voltage shift on output the entire chain.

 

[KlausST]

Hi,

offset voltage on on each amplifer make voltage shift on output the entire chain.

True, but this can simply cancelled out with a blocking capacitor, as long as you don´t need the DC information..

So the question still is: Do you need to measure the DC voltage at the output of the chain?

Klaus

 

[Audioguru]

Your attachment does not work so we do not know the function of the circuit.

The AD8610A low noise opamp has a typical open loop gain of 250 at 100kHz. Is it needed to reduce 100kHz distortion? It costs 2.8 times more than an OPA134 audio opamp that also has low noise and has an open loop gain of 100 at 100kHz where its distortion will be about 0.01% with the gain of 20x. Is that good enough?

 

[berger.h]

OK I resent attachment
View attachment IA147.png
Suppose
On + input Instrumetation amplifier is voltade from +-12,5uV to +- 1,25uV sinus max frequency 100kHz
On - input Instrumetation amplifier is voltade near 0 but not nul may be feww uV max +- 1,23mV
ISS this voltage amplifi 1x or max 20x
Secon stage - 4th LPF on 100kHz reduce energy on noise between 100kHz and 5MHz
Third stage is inverting amplifier and level shiffter, who make from +-1,25V 0-2,5V and drivimg ADC , may be 14bit 10MSPS.
We also measure phase shift, so the amplifier must significantly and randomly change the phase!

What's discussion?
Found in the first case will allow the entire process chain DC voltage.
(C5 is clamped)
Important feature of the amplifier becomes input offset voltage and his temperature shift.
According to my, this discriminates amplifier as opa134, because +-3mV input voltage offset make on 4pcs amplifier error - 12mV
without requiring DC. Is possible add before last amplifer blocking capacitor and input voltage shift will be of interest only at the last amlifier.
I simulate thiw circuits with AD8620 and the overall accuracy was acceptable (Pricing has not) :grin:

 

[KlausST]

Hi,

according this informations and the informations of post#2..what opamp did you choose?

--> got to a dsitributor (where you want to buy from) internet site and use the OPAMP selection guide.
--> Or go to an OPAMP manufacturer internet site.

Klaus

 

[Audioguru]

An instrumentation amplifier is used when the input signal is balanced and you need very high input resistances. But your input signal is not balanced so an ordinary non-inverting single opamp can be used.

 

[berger.h]

Thank you for all the answers.
Yesterday I was a little simulated :x (in multisim)
signal source 1Vpp 100kHz, amplifier as buffer and measured THD on 100kHz 1-10th harmonics FFT 4096 point

Amplifier	BW	SW	THD
AD8620	25MHz	50V/us	<-100db
ADA4896	230MHz	120V/us	<-100db
AD8597	10MHz	14V/us	<-100db
MCP6021	10MHz	7V/us	        <-100db
ADA4805	40MHz	190V/us	  -100db
AD8661	4MHz	        3.5V/us	  -76db
OPA227	8MHz	        8V/us	          -71db
ADA4661	4MHz	        2V/us	          -65db
AD8638	1,35MHz	2,5V/us	  -59db
ADA4627	19MHz	82V/us	  -27db

I fail to see what parameters affect THD? For example ADA4627 is very precise amplifier and THD on 1kHz is 0.000045% and SPICE model on 100lHz say THD 4.6%
Why?

 

[Audioguru]

Either the datasheet is wrong (I doubt it) or Spice has an error for the model (probably).