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XTR111: Dac voltage drops when supplying to xtr111

Part Number: XTR111
Other Parts Discussed in Thread: LM358, LMC7101, LM2674

Hi experts.  

This time i am using xtr111 to generate 0-20mA signal corresponding to the relative humidity in the environment (as per the datasheet of xtr111). Used SHT41 (humidity sensor from sensirion) a microcontroller and a dac (mcp4725). When tested for 0-3.3V signal that i had programmed for MCP4725 without taking xtr111 circuitry in picture everything goes well. The sensor shows RH as 64% which is matching to a reference humidity sensor.

When I include xtr111 circuitry in the picture the voltage from the DAC(mcp4725) drops to almost 1/3. I assumed that xtr111 is sourcing more current from the DAC and applied an opamp(voltage follower using LM358) but it gave me no luck. 

Now can you please suggest me any solution ?? 

Rset i had used as 1.65k to generate 20mA at full scale voltage of 3.3V.

Do i need to use another opamp, other than LM358??

If yes, can I use LMC7101 (because i have it readily available with me)??? 

Or is there any other way out..?? Please help. 

Thanks in advance. 

  • Hi Chetan,

    a schematic would be helpful.


  • Hi.. 

    Ok.. I'll just upload the schematic.. in few minutes



  • Ok I found something.. my PCB is heating up.. as i am using 24V as input, the regulator 7812 and 7805 both are getting heated and this heat is i guess the reason behind signal drop (which is actually the drop of sensor reading and not any voltage drop of the DAC.

    What i did is, i supplied the circuit with 12V as input rather than using 24V. This time, the readings were not 1/3rd as the heat was less.. i used a fan to cool the pcb and it also helped and readings were very much in the proximity of the original humidity reading in the reference sensor.. but still the 7805 is getting heated and affecting the reading little bit.. 

    Now the complete discussion is about how to reduce the heating of the regulators..?? Is there any other solution to obtain 5V from 24V without heating the circuit..??

    Thanks in advance.. Blush

  • Hi Chetan, 

    I have a couple of questions/comments regarding the  LMC7101 (op-amp) and XTR111 circuit:

    - Per your comment above, the expected DAC signal is 0 to +3.3V, and the schematic shows the XTR111 is configured with RSET = 1.65kΩ resistor to produce a 0-20mA output signal, when applying a 0-3.3V input voltage.  However, the LMC7101 op-amp in the schematic is configured with a non-inverting gain of 1+62k/120k =  +1.516V/V.   Please note that the LMC7101 will be slammed at the +5V rail (the output swing is limited to around ~4.9V while powered with +5V supply) when applying a 3.3V DAC signal.   Did you meant to configure the amplifier as a buffer with Gain of +1V/V? Please reduce the amplifier gain, or may need to reduce the DAC input voltage range. 

    - The XTR111 REGF and REGS pins are left disconnected on the schematic.  When not using the XTR111 voltage regulator, the recommendation is to connect the REGF and REGS together, and use a 470nF capacitor.  Please see Figure 45 (a)  on p. 18 of the XTR111 datasheet:

    Please note, the L7812 and MC7815 voltage regulators are not TI devices.  Please consult directly with the voltage regulator manufacturer for support / recommendations on these; or kindly clarify the part numbers if you are using TI devices

    Thank you and Best Regards,


  • Hi Chetan,

    What is the part number for the Q3 MOSFET P channel transistor? 

    Another recommendation is to ensure to use a proper heatsink on the P-FET Q3 to avoid heating issues. Please refer to the XTR111 board layout on page 7 of the XTR111EVM User's Guide, the drain of the P-FET (pin2, pin4 heat sink) are connected to an isolated copper Fill on Top and connected to another isolated copper fill on the bottom for heat dissipation. Please see figure below.

    If you wish, we could review your PCB layout, please provide the Gerber files.

    Thank you and Best Regards,


  • Hi Chetan,

    Now the complete discussion is about how to reduce the heating of the regulators..?? Is there any other solution to obtain 5V from 24V without heating the circuit..??

    We use the LM2674 as a pre-regulator for the LM7805 and decrease the +24V input voltage to +8.5V. Then the LM7805 has no longer to dissipate 24V - 5V = 19V but only 8.5V - 5V = 3.5V. This is less than 1/5 Relaxed


  • HI Chetan,

    Please let us know if you have further questions related to the XTR111 and LMC7101 amplifier circuit.  

    For voltage regulator specific questions, it may be best to submit a query to the Power management forum, where the power management applications team will be able to offer alternative solutions.

    Thank you and Regards,