This is my schematic

Hi,

you could do it this way:

Kai

thanks Kai.

What's wrong with my circuit? Please can you answer? Why is the output of the opamp about 0.0015v when the opamp is not connected to the XTR111 and I apply 0V to the opamp input, but 0.65V when I connect it to the XTR111?

HI Elektroboard,

I am unable to open your schematic pdf file. 

The post above provides a link to a share box, and TI’s network access to cloud file sharing sites is restricted.

Can you please re-attach your schematic pdf file directly to the E2E post by using Insert - Image/video/file - Upload options on the menu below?

Thank you and Regards,

Luis

ADC_modul3.pdf

Hi Luis. I inserted the schematic. Thanks for your informatin.

Hi,

The operational amplifier output requires an amount of voltage headroom to the supply rails to work within its linear region. The TLV2170 requires more than 0.2V headroom from the negative supply rail for output swing headroom.  The datasheet offers a more conservative spec of VEE+0.35V to be within its optimal linear region:

Hence the amplifier will be outside its linear range when the DAC output is at 0V, while powered with a unipolar supply.

Kai has suggested a circuit using the LM7701 negative bias generator, to produce a -232mV negative supply to power the op-amp circuit.  In addition, the op-amp was replaced with the LM358.  This amplifier requires a smaller voltage headroom of 100mV output swing from the  negative supply rail.

Another possibility is the TLV9302 as shown on simulation below.  The simulation below assumes that the DAC output range is 0-1V, the second stage has a non-inverting gain of 10-V/V, and the op-amp circuit is powered with a 12V supply and the LM7701 -232mV bias generator.   The XTR111 voltage regulator is used to generate a 10V DC signal.  The resulting input voltage at the XTR111 input is +2V to +10V for a 4-20mA output.

The gain of the second stage and/or XTR111 circuit can be adjusted for different DAC output voltages.

Thank you and Regards,

Luis

Hi,

Please see below the simulation results.

Note:  The simulation below uses an idealized/approximate XTR111 model, the XTR111 circuit attached does not simulate the XTR111supply headroom limitations, AC/bandwidth and noise performance of the XTR111. However, this ideal XTR111 model could be used as a tool to quickly verify the input/output ideal transfer function.

TLV9302_XTR111.TSC

Thank you and Regards,

Luis

hello luis, thank you very much for your replies. I'm happy. I will reconsider your answers and my problem. but when I connect the su:opamp output to xtr111, I get about 0v . but when I connect opamp to xtr111, opamp output becomes 0.65v. Why is the value degraded? 

Hi,

This is because the OPA2170V output of the first stage buffer is unable to reach 0V while powered with unipolar supply, where the negative supply is connected to GND. Essentially the first op-amp output is at a voltage somewhere around ~100mV to 200mV above ground, with the amplifier outside its linear region. 

The OPA2170 amplifier output swing is limited to more than +0.2V voltage above the negative supply. In other words, the amplifier output swing requires more than 200mV headroom above the negative supply rail. The datasheet recommends allowing a conservative 350mV headroom to place the amplifier within the optimal linear region.  The second stage will gain the first amplifier signal, which is at a voltage above GND, and produce a voltage output.  Also, the second stage non-inverting gain stage circuit also appears to be incorrect.

As we suggested, if you intend to use the OPA2170, you will need to power the amplifier circuit using bipolar supplies, where V+ positive supply has to be more than >+12V and V- (negative supply) needs to be more negative than VEE<-0.350V.  You will keep the XTR111 powered with GND and the V+ supply.  

Alternatively, you could use the LM7705 to generate a negative supply of -232mV; and choose the LM358 or TLV9302 or another op-amp that offers output swing to ~100mV from the negative supply.

If you would like to learn more details about op-amp input-output range limitations, please review the TI Precision Labs tutorial below:

Operational Amplifier input and output limitations

Thank you and Regards,

Luis

hi Luis. Thanks a lot for the detailed explanation. I understand the issue better now. I guess I need to remove the 32k resistor in the circuit diagram, right? Also, I need to set R55 47k to 40k (as in the dtasheet). Are these values ​​so critical?

Corrected image above...

HI,

Thank you,

Best Regards,

Luis