Op amp configuration

[pacman22]

What configuration is this op amp in? VREG is supplied with half of V1 i.e, 2.5V.

 

[danadakk]

Its Non Inverting, single supply, biased to Vreg. Saying this because no apparent
input G setting R on inverting input.

But then with no input G setting R could be an a TIA current input configuration
fed by CH1.

Basically something like this (only Vee = ground, input is a photo diode current) :

Regards, Dana.

 

[KlausST]

Hi,

@OP:
Where is this schematic from?
Doesn´t it come with some infromation?

Klaus

 

[andre_luis]

The circuit seems rather simple, perhaps wouldn't take long to infer its characteristic Vo/Vi transfer function.

 

[FvM]

Makes sense to analyze the circuit as TIA (transimpedance amplifier). T-feedback circuit to avoid a 60 Mohm resistor. Guess you wanted to say it's inverting. I would prefer to sketch the signal current source to clarify the function.

--- Updated Oct 11, 2022 ---

By the way, according to datasheet AD711 won't work with 5V single supply. Minimal supply voltage +/- 4.5V, negative Vcm limit Vs+4V.

 

[pacman22]

Hi,

@OP:
Where is this schematic from?
Doesn´t it come with some infromation?

Klaus

The schematic is a part of a signal conditioning circuit. The input to this op amp would be from a hydrophone.

 

[KlausST]

Hi,

please post a link to the source of the schematic.

Klaus

 

[FvM]

O.k., your hydrophone is apparently a capacitive sensor, most like a piezo transducer. I presume AD711 isn't used with 5V supply in the real circuit, you have placed it in your LTspice circuit because it's available in the default library. However, if AD711 common mode voltage range is correctly modelled (it's not necessarily the case), the simulation would fail. Better use a 5V compatible OP.

Have you any open questions beyond the points that have been already answered?

 

[pacman22]

Have you any open questions beyond the points that have been already answered?

1. R2 and R3 seem like a voltage divider. What is their purpose here?
2. Op amp's non inverting terminal if fed with 1/2 of V1. Why so?

 

[FvM]

1. Yes, a voltage divider, replaces a 60 M feedback resistor, as stated. Disadvantage is higher resistor noise and amplification of OP offset voltage.
2. Typical bias circuit for single suppply OP.

 

[pacman22]

By the way, according to datasheet AD711 won't work with 5V single supply. Minimal supply voltage +/- 4.5V, negative Vcm limit Vs+4V.

How did you find this? Where should I look for this information in the data sheet?

 

[FvM]

Supply voltage range, input voltage respectively common mode voltage range.

 

[KlausST]

Hi,

How did you find this? Where should I look for this information in the data sheet?

It seems you are not that experienced in reading datasheets, thus I tell how I do it:

There are some important basic informations. Usually on the first page:
* the headline (gives basic, but still important informations about the device)
* the feature list (just shows some key specifications. Often shows where "this" device has improvements against competitor devices)

Then there are two chapers of technical specifications. (important)
* typical / recommended operation specifications (typically as table. Usually more at the end of the datasheet).
It gives all necessary informations you need to operate the device. There are sections for: Inputs, Outputs, Supply and other external / internal things. If you go beyond the given limits: in best case the device operates normally but with increased errors. But it also may fail to operate completely.
* Absolute maximum ratings. Don´t normally operate your device at these limits. Going beyond these limits you may immediately kill your device. Or it may harm your device and cause long term failure.

Example: You desing a circuit used in a car.
The "nominal car voltage" is 12V. But this value is almost useless information. What you need to know:
The nominal voltage range if a car is 10.5V(cold, weak battery) to 14.4V (fully charged). And you need to know that there may be spikes/transients maybe up to 30V (depends on situation, maybe even higher)
So you need to look for a device with normal operation range of "10.5 .. 14.4V plus some headroom, but it needs to withstand voltages up to 30V. If your devie can´t stand these 30V you need to install additional protection circuitry that limits the voltage to a lower value, matching your device specifications. (But indeed designing electronics for cars can fill a whole book...)

I can only recommend to use the search function of your PDF reader:
So if you are concerned about "noise", then do a search about "noise". Start your search at the beginning of the section you expect the information. The technical limits are shown in the specifications table, but also important information is given in the textual description section.

Example: "How do I find this":
* go to the section of "recommended operation specifications" (beginning of table)
* do a search for "supply"
* step through the results until the table entry headline "POWER SUPPLY" (or similar)
--> there you are.
Additionally I recommend to use the keyword "supply" and do (fast) search through the textual descriptions.
focus on chapter headlines containing "supply", but also do a quick check on every hit if there is important informations. Often you find recommendations about proper decoupling of the supplies and maybe even example schematics and/or PCB layout recommendations.

--> I use the PDF search very often.
Example: You look for "supply" then you get hits for "POWER supply". You may do a new search for "POWER".
In the above datasheet you get a hit at fig.1 where it shows that (for the given schematic) the star point for GND should be at pin 12 of AD565. Maybe important for your application, maybe not.
But at least a reminder: Should I also care about the star point in my application, too?

Klaus

 

[pacman22]

Thanks for your answers! I still cannot figure out the purpose of R1 and R2 here.

 

[KlausST]

Hi,

How I see R1, R2, R3 regarding gain: (ignoring the DC offset)

R2 and R3 form a resistive divider 101:1
and the R1 is now the feedback from this "V_out / 101" node.

in values:
1V of change in output
causes about 0.01V change in voltage at common node of R1, R2, R3
and this causes 0.01V/604k = 16.5nA of change in feedback current.

Referred to the output it acts like a single feedback resistor (from OUT to -IN) with 1V/16.5nA = 604Mohms

It´s hard to find (precise) resistors in the range of 600 MOhms.

****
Now about offset:
R3 makes the output operation point to be about VRef.

The disadvantage of this circuit: It amplifies the OPAMP´s offset_voltage_error by a facor of about 100.

Klaus

 

[FvM]

 

[pacman22]

2. Typical bias circuit for single supply OP.

I didn't quite understand this. Can you explain a bit more on this? What would happen if the non inverting terminal is tied to ground?

 

[FvM]

You should ask yourself, what's the signal source output current range and intended output voltage range of the amplifier?

Single supply OP limits the output voltage range to positive range, most negative output is 0 + output saturation voltage. I presume your hydrophone is sourcing AC current, it can be only reproduced by the amplifier if it has a bias point somewhere in the middle of the output voltage range.

 

[danadakk]

SIM does not seem to make sense either ? But I think its correct, assuming the input
is a current source. But clearly the OpAmp was chosen for its very high Zin.....Clearly
OpAmp does not have much CM range with a 5V supply.

Regards, Dana.