• State
  • Locked Locked
  • Replies 5 replies
  • Subscribers 48 subscribers
  • Views 257 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMH6505: Input voltage range adjustment using Rg

kaveh gharehbaghi
Prodigy 10 points
Part Number: LMH6505

Hi

The datasheet of LMH6505 explains that RG determines the input voltage range. I struggle to understand how a resistor which is connected to the output of buffer sets the input voltage range. Can you explain it in further detail?

Also what is the acceptable range for RG and what is the trade-off in increasing the RG? Datasheet says we can keep it open to get +/-3V input range at the cost of lower max gain. However, in my test setup I couldn't go beyond 1kohm (no output when RG>1kohm).

Again, according to datasheet, we should only loss a little bit of gain by increasing RG but my intuition based on measurement tells me it is more than that because I couldn't get the output with RG open (while having nice output at RG<1khom).

Thanks

Kaveh

  • 0
    TI__Mastermind 26595 points

    Hello Kaveh,

      That is a good question. The system block diagram showed in the datasheet in Figure 41 should have a connection to the MULT block like shown in Figure 3. Otherwise, you are correct, it would not have an impact.

      Increasing RG should lower max gain, but increase input voltage range. The range for RG would be less than 100 ohms. That is strange, RG can be left open according to the datasheet, which would give you the max input voltage range of +/-3V/V. 

      Would you be able to share your test schematic?

    Thank you,

    Sima

      

  • 0 in reply to Sima Jalaleddine
    Prodigy 10 points

    Hi Sima

    Thank you for your response. The schematic is as shown bellow. Beside the table in datasheet that suggests Rg can be open, the value of RG in figure 6 is 510 ohm. So the datasheet doesn't rule out the possibility of setting the value more than 100 ohm. I want to know how large it can be and what is the trade-off? Right now I set it to 750 ohm to accommodate the range of input signal that I apply. This is design for product and I need to make sure it is safe to set it to 750 ohm because the datasheet shed little light on the effect of high RG. The other reason for being concerned is that I couldn't get the output with RG open so I definitely need to know how large it can be.

    Thanks

    Kaveh

  • 0 in reply to kaveh gharehbaghi
    TI__Mastermind 26595 points

    Hello Kaveh,

      Thank you for sharing the schematic. You are correct, it does look like datasheet specifies higher RG. I will look into this, and get back to you with more information. 

      Not too sure if it would change much, but would you be able to short pin 4 straight to ground as a test (remove C27 and 0 out R136). 

      Do you need to go above >1kOhm for RG? Are you also changing the gain control and/or RF as you increase RG? 

    Thank you,
    Sima

  • 0 in reply to Sima Jalaleddine
    Prodigy 10 points

    Hi Sima

     connecting pin 4 straight into the ground doesn't change the outcome. I am ok with 750 ohm ( no need to go beyond 1k). But I need to know this is the safe zone for the device. Again, this is design for production and I need to make sure RG~1k  is ok for all devices. If necessary please include and application engineer to expedite answering the issue.

    Thanks

    Kaveh 

  • 0 in reply to kaveh gharehbaghi
    TI__Mastermind 26595 points

    Hello Kaveh,

      Thank you for trying out the suggestion. I am asking a design engineer on the team if there is a max RG. I would think this value would be somewhere in the kOhms, but not "open" or floating as suggested in the datasheet. 

       When you are increasing RG are you also increasing RF? Otherwise, you will run into stability issues if you go below a gain of 0dB or 1V/V. But, usually CFAs need to be set at a slightly higher gain than unity, and as stated in the datasheet that we recommend Avmax to be set between 2 to 100V/V. The multiplier k of 0.94 might be causing an issue where your gain will fall below unity. Can you increase RF to 5kOhms? I am an application engineer, but I will let you know when our design engineer can look into the max RG setting. 

    Thank you,
    Sima