OpAmps and Circuit type

[drkidd22]

I brief description would be nice. Not sure how exactly R244 works here or if I even need it.

 

[spec]

I brief description would be nice. Not sure how exactly R244 works here or if I even need it.

OK will do- I should be able to explain about R244. I am on social duties tomorrow (2016_06_22) though.

spec

 

[MikeMl]

Anybody have a data sheet of the opto-detector? I found similar that are "linear" and others that are "digital". I would really like to know the CTR (current transfer ratio)?

 

[spec]

Here is an explanation of your circuit. Below is a redrawn and rationalised schematic. It looks like R278, shown as 290 Ohms on the original schematic should read 390 O

Issue 6 of 2016_06_24

(1) ERRATA
(1.1) Insert R258 (10K) between the 10V supply rail and the non-inverting input of the opamp.

(2) CIRCUIT PURPOSE
The purpose of the circuit is to produce an output voltage proportional to the opacity of the medium in the opto slot- the more opague the higher the output voltage.

(3) CIRCUIT FUNCTION
U52A is a standard precision FET input opamp (OPA2604) configured as an inverting virtual earth (ground) amplifier, effectively operating with supply lines of 14V, 0V, and -10V (+24, 10V and 0V).

The fundamental function of any opamp is to make its non-inverting input and inverting input the same. It will try to achieve this by adjusting it's output voltage. This is a set-in-concrete rule about any operational amplifier functioning in the linear mode. Because of this, both opamp inputs, of the above circuit, will always be 10V as defined by the 10V supply line which is connected to the opamp non-inverting input (due to the vanishingly low input bias current of the OPA264, R258 has no impact on the voltage of the opamp non-inverting input).

The more opaque the material in the opto slot the less current will flow to the non inverting input through RR245. This means that the non-inverting input voltage will drop. This in turn will cause the opamp output to rise. This in turn will cause the the current through the opto LED to increase, which in turn will cause the LED light intensity to increase which in turn will cause more current to flow in the opto receiving transitor untill the two opamp inputs are the same voltage, when equilibrium will be achieved.

R244 is only for biasing. R244 sinks 10V/R244 500uA current away from the non-inverting input of the opamp which causes the output voltage to be higher by a constant. As the non-inverting input of the opamp is always at 10V the current through R244 never changes and just forms a positive voltage offset on the output.

The gain of the opamp is R235/R245 and R244 has no impact on the gain.

Capacitor, C91, across R235 forms a first order low pass filter which slugs the opamp response and attenuates any mid and high frequency noise from the input to the output. The corner frequency of the low pass filter is when the reactance of the capacitor equals the resistance of R235. Thus fb= is found by the formula 1= R235/(2*Pi * fb *C91)

For some reason there, is a 4 to 1 attenuator and negative voltage offset formed by R277 (3K) and R276 (1K) on the opamp output. This seems odd because the same function could be implemented by changes to the values of R244 and R235. The two resistors do provide isolation from the opamp output and generate an output impedance of R277 in parallel with R276.

R258 (10K) serves no purpose.

R22 (390R) has no influence on the fundamental operation of the circuit. It's only function is to reduce the dissipation in the opto reciever transistor.

C97 (100nF) is a decoupling capacitor from the 24V supply to 0V. A similar decoupling capacitor should have been included between the 10V supply line and 0V. Both capacitors would reduce the impedance of the supply rails which helps ensure that the opamp is stable in the frequency domain.

spec

 

[drkidd22]

Thanks a million. I have a better understanding how this works.

 

[spec]

Thanks a million. I have a better understanding how this works.

No probs- I also have a better understanding about how the circuit works too.

spec