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# Op Amp selection and support help

Hi ,

We‘d like use resistor networks and operational amplifiers to monitor the voltage state over telephone lines. Please see below

The voltage on the telephone line, Vtip-ring, is generated by a voltage generator. The Vtip-ring usually has the following values

1.Vtip-ring =0V Indicates that there is no electricity

1. Vtip-ring= +10Vdc or -10VDC Indicates that the phone is hang off
2. Vtip-ring=+48Vdc or -48 VDC indicates that the phone is hung on
3. Vtip-ring= +120V or -120V Indicates that the phone is ringing.

In schematic , measuring vTIP-Ring at high resistance values through resistance networks R1, R2 and R3, and then convert it to voltage values through operational amplifiers . Low accuracy can meet the requirements of distinguishing the above four telephone line states.

The voltage on the phone line, vtip-ring, is usually dc and ac (50Hz Sine wave) when ringing.

My question is

1. Can you recommend a single-ended rail-to-rail operational amplifier to meet the needs?
2. The voltage generator is connected to a different ground than the operational amplifier.The power supply of the op amp is 5V.In this application, do we need to consider the common mode voltage range of the op amp?
3. Sometimes Vtip is greater than Vring (Vtip-ring>0, such as +48V), sometimes Vtip is less than Vring (vtip-ringn>0, such as -48V), how can Vcc/2 be used as the middle voltage in the single-end output of op-amp and be judged as if the value is less than Vcc/2 ,it can be judged as Vtip-ringn<0;

If the value is greater than Vcc/2, it can be judged as Vtip-ring>0?

Thank you.

• Hi szwx70,

Can you recommend a single-ended rail-to-rail operational amplifier to meet the needs?

Analog Devices has a wide profile of rail-to-rail amplifiers. You can check this op-amp parametric search for a quick overview of the available rail-to-rail amplifiers with 5V supply.

The voltage generator is connected to a different ground than the operational amplifier.The power supply of the op amp is 5V.In this application, do we need to consider the common mode voltage range of the op amp?

I think this won't be an issue for the common mode voltage since the ground of the voltage generator is also just a typical ground just isolated (correct me if I'm wrong with this) and the op-amp will still also refer to a typical ground.

Sometimes Vtip is greater than Vring (Vtip-ring>0, such as +48V), sometimes Vtip is less than Vring (vtip-ringn>0, such as -48V), how can Vcc/2 be used as the middle voltage in the single-end output of op-amp and be judged as if the value is less than Vcc/2 ,it can be judged as Vtip-ringn<0;

The most basic thing to do here is connect a common-mode voltage to your input that is basically half your Vcc since op-amps don't have pins for the Vcm.

In order for us to suggest a more specific amplifier for your application, could you share more details:

1. What is the purpose of R3?

2. What is your target gain?

3. Do you have  other specs that you need to consider? (noise performance, dynamic performance, etc.)

4. If its not too much to ask, could you share your actual/working schematic if you have one so I could try to simulate it on my end.

Thanks and regards,

Kristine

• Hi Kristine，

Thank you very much for your reply. I would be happy to send schematic files and simulation files to you. Originally, I wanted to send the file to you through Forum, but I operated it for several times and upload failed. If you tell me another way to transfer the document, I will send it to you as soon as possible. Below is the schematic diagram and multisim simulation file I sent to you.

PDF

Let me outline the features I want to implement. The battery in the remote equipment room supplies power to the PHONE through the telephone line. The actual voltage on the phone line can occur only in the following ways

0V,+/ -10VDC,+/ -48VDC,+/ -120V 50Hz AC. Therefore, the bandwidth requirements of the operational amplifier can be ignored, and the accuracy requirements are not high.

I want to measure the voltage on the telephone line with a single-ended op amp. But do not want the common-mode component of this voltage to operate beyond the operational amplifier's operating range. Otherwise I might have to consider higher cost isolation op amp.

Here is my general description of the schematic

The Vdm is a DC voltage source that simulates the maximum differential voltage of the remote battery on the telephone line. The Vcm is also a DC voltage source that simulates the common-mode voltage diffrence of the remote equipment room GND and the measuring end GND (in normal cases, the Vdm is far away from the measuring end R1 and R2. It's 1-2km , so I think there's a CM voltage difference).R1, R2, R3 act as resistance dividers to isolate the telephone line from the back-end measurement. At the same time, the differential mode voltage on the line is measured to reduce the signal amplitude input to the operational amplifier.

R4.R7,R5, and R6 together form a differential amplifier with the OP amp, measure the voltage on the phone line through the OP amp output (the latter stage is sampled by the ADC).As you suggested, I used R9 and R8 as VCC / 2 generators. By connecting R7,Raise output potential 0 to VCC / 2;(ORIGINALLY I wanted to further design the VCC down to 3.3V in order to use a 3.3V ADC for sampling).But now I have some problems .Under simulation, I found the following problems.

1. I would like to adjust the gain proportional resistance (R4/R7, R5/R6) rather than by adjusting the isolation voltage distribution network (R1, R2, R3), VCM =+ -500V, VDM=+-120 state, the +, - input voltage of the op-amp should not exceed VCC , In this way ,Even if the common mode ground Voltage difference at the two end of the telephone line is 500V, at the maximum VDM, the op amp will not cause distortion due to overvoltage range input. However, no matter how to adjust R4/R7 and R5/R6, I can only guarantee that Vcm=+ -350V to meet this requirement.Is there something wrong with me?
2. I would like to adjust the gain proportional resistance (R4/R7, R5/R6) instead of the isolated voltage divider network (R1, R2, R3) so that at maximum Vdm (+ -120v), the op-amp output is just near the power rail (i.e

When Vdm is 120V, the op-amp output is close to VCC, and when Vdm is -120V, the op-amp output is close to 0.This can improve the accuracy of the measurement. However, the simulation results show that when Vdm is 120V, the op amp outputs VCC/2+0.6V, and when Vdm is -120V, the op amp outputs VCC/2-0.6V). Did I do something wrong? Or did the emulated op not reach track-to-track (I chose 3288RT in Multisim)? Can you provide a suitable op amp model that you can emulate in Multisim?

Br,

Carson

• Hi Kristine，

Thank you very much for your reply. I would be happy to send schematic files and simulation files to you.  Below is the schematic diagram and multisim simulation file I sent to you.

PDF

Let me outline the features I want to implement. The battery in the remote equipment room supplies power to the PHONE through the telephone line. The actual voltage on the phone line can occur only in the following ways

0V,+/ -10VDC,+/ -48VDC,+/ -120V 50Hz AC. Therefore, the bandwidth requirements of the operational amplifier can be ignored, and the accuracy requirements are not high.

I want to measure the voltage on the telephone line with a single-ended op amp. But do not want the common-mode component of this voltage to operate beyond the operational amplifier's operating range. Otherwise I might have to consider higher cost isolation op amp.

Here is my general description of the schematic

The Vdm is a DC voltage source that simulates the maximum differential voltage of the remote battery on the telephone line. The Vcm is also a DC voltage source that simulates the common-mode voltage diffrence of the remote equipment room GND and the measuring end GND (in normal cases, the Vdm is far away from the measuring end R1 and R2. It's 1-2km , so I think there's a CM voltage difference).R1, R2, R3 act as resistance dividers to isolate the telephone line from the back-end measurement. At the same time, the differential mode voltage on the line is measured to reduce the signal amplitude input to the operational amplifier.

R4.R7,R5, and R6 together form a differential amplifier with the OP amp, measure the voltage on the phone line through the OP amp output (the latter stage is sampled by the ADC).As you suggested, I used R9 and R8 as VCC / 2 generators. By connecting R7,Raise output potential 0 to VCC / 2;(ORIGINALLY I wanted to further design the VCC down to 3.3V in order to use a 3.3V ADC for sampling).But now I have some problems .Under simulation, I found the following problems.

1. I would like to adjust the gain proportional resistance (R4/R7, R5/R6) rather than by adjusting the isolation voltage distribution network (R1, R2, R3), VCM =+ -500V, VDM=+-120 state, the +, - input voltage of the op-amp should not exceed VCC , In this way ,Even if the common mode ground Voltage difference at the two end of the telephone line is 500V, at the maximum VDM, the op amp will not cause distortion due to overvoltage range input. However, no matter how to adjust R4/R7 and R5/R6, I can only guarantee that Vcm=+ -350V to meet this requirement.Is there something wrong with me?
2. I would like to adjust the gain proportional resistance (R4/R7, R5/R6) instead of the isolated voltage divider network (R1, R2, R3) so that at maximum Vdm (+ -120v), the op-amp output is just near the power rail (i.e

When Vdm is 120V, the op-amp output is close to VCC, and when Vdm is -120V, the op-amp output is close to 0.This can improve the accuracy of the measurement. However, the simulation results show that when Vdm is 120V, the op amp outputs VCC/2+0.6V, and when Vdm is -120V, the op amp outputs VCC/2-0.6V). Did I do something wrong? Or did the emulated op not reach track-to-track (I chose 3288RT in Multisim)? Can you provide a suitable op amp model that you can emulate in Multisim?

Br,

Carson