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Part Number: INA125
For the last six months, I have been using INA 125 P amplifier. The problem I am facing is that in figure 6, page number 13 in the datasheet, pins 14 and 4 have been connected and due to this connection, we would be getting a reference voltage of 2.5V (as is shown in the figure). Experimentally, I also made the same connection but I am getting a reference voltage of 1.44V. Why this anomaly is happening when everything is correct in my circuit? Please help.
In reply to CAPTAIN SWING:
Zak Kaye Precision Amplifiers Applications
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In reply to Zak Kaye:
After yesterday's reply, I tried with a different transistor in place of TIP29C. The transistor that I used was 2N2222. My first question is that I couldn't understand the circuit diagram of figure 4, involving the transistor. (which I have attached herewith). Emitter and base portion is clear but in the collector region, only V+ is written. I want an excitation voltage of 2.5V. So, what should the appropriate value of V+. and where to connect V-? Is V- connected to the pin 4 as shown by blue line in the diagram?
First of all, thank you very much. You had been replying all my queries one after the other.
I have connected the V- of the power source to the ground and the V+ to the collector as shown in figure 4 of INA 125 datasheet. So, now, I get the excitation voltage as is given in the datasheet. i.e.(2.4786 volts)
But now, I face another problem, my Rg (between pins 8 and 9) is 1kilo ohm. So, I get an amplification factor of 65. But when I make Rg equal to 120 ohm, I do not get the amplification factor of 500. It gets saturated at 1.18 volts. My input voltage to the amplifier is 11mV. So, after amplification, the voltage should be 5500mV but what I get is 1180 mV. My supply to the INA 125 and transistor is 5 volts. Why is this happening? Please help.
The signal that I want to amplify is a dc input signal. So, I think there won't be any question of bandwidth for my problem as frequency is zero Hertz. I didn't understand the second and third lines of your reply.
So, once again, I am just telling you that my Rg is 121 ohm. So, AF is 500. My input voltage to the amplifier is 10-12 mV DC. So, after amplification, the voltage should be 5000-6000 mV DC. But the voltage that I get after amplification is less than the theoretical value. The supply to the transistor and the amplifier is 5 volts.
Just I would give an instance.
Input voltage to the amplifier=10 mV DC
Output amplified voltage=3.1 V DC
For this instance, I would just give the voltages of each pin with respect to ground
So, can you suggest me how I can get the desired output amplified voltage that matches with the theoretical value.
The connections are all same as discussed in the previous posts.
There are a couple issues here and I will try to explain as best I can: 1) The voltage you have measured at pin 5 suggests you have not tapped the transistor at the emitter, but rather the output of the reference voltage op-amp. The 2.5V transistor output is the accurate reference that you want to use. However, if you are only applying a dc signal that will never change polarity, you don't even need to bias the output to a pseudoground as done in figure 6, and you can have your load reference to ground. See below for a comparison: 2) You can't expect your output voltage to exceed, or even equal your supply voltage. No op-amp can truly swing all the way to the supply rail. Biasing pin 5 means that pin 10 must go to IAref + 5V, which means you need an even greater supply voltage. So if you have 2.5V at IAref, then the output needs to swing to 7.5V to produce a 5V differential output. On top of that, worst case the INA125 can only swing to within 1.2V of your positive rail, meaning in that same scenario you would actually need a supply voltage of 8.7V to get a 5V output. The short answer is that if you increase your supply voltage to about 10V, then you shouldn't have any problems.
I made the necessary corrections like connecting pin 5 to the emitter of the transistor, so that the pin 5 is always at a voltage of 2.5 above the actual ground. I also could understand the important facts in your last reply. But now the problem is that I am able to get an amplification factor of 65 and 500 but not 1000. I would give three instances-
For all the three instances, supply voltage to the amplifier is 15.6 volts and to the transistor is 5.2 volts.
Similarly, for a gain of 500
For a gain of 1000,
In the second and third instance, we can see that the measured amplification factor is in great variance from the theoretical amplification factor. If saturation of voltage was to take place, then it should have started at 10 volts or so but why in third instance, the voltage gets saturated at 6.008 volts
My main purpose is that I want to amplify the signal into 100 times or 1000 times. Please help why saturation takes place so early. In the data sheet, it is written that we can amplify to an extent of 10000 but here getting a gain a consistent gain of 500 and 1000 is becoming difficult.
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