TLV9004: Input Impedance(Resistance) information

Hi Jaehoon,

I am not sure that this data was taken.  Do you know how high your input impedance needs to be?  Will the typical input impedance on the order of 10^12 not be large enough?  Usually, this is large enough so as to not be a concern.  Instead, we worry about the input bias current of the amplifier.

There are no standard op amps in an 8-channel package.  However, you may poke around and find some other TI products that may fill your need.  Perhaps a product such as the VCA8500 would do?

Let me know your thoughts.

Regards,

Daniel

Hi Daniel,

Thanks for your reply.

If the input impedance of TLV9004 is on the order of 10^12, I think it would be fine.

What would be the potential issue of high input bias current? I checked that TLV9004 has 5pA input bias current. If it is too high for the microphone preamp application, then I think I can use TLV9604 which has 0.5pA input bias current. However, I would like to know the potential impact of high input bias current on my application. 

Also, I checked VCA8500 but its input resistance is 8kΩ which is way too low for preamp application.

Regards,

Jaehoon

Thank you very much for the further explanation Kai!

Jaehoon,

As Kai has pointed out, you can use figure 7 to estimate the "common-mode input resistance."  The exact number you get through your linear estimate should not be as important as understanding the magnitude of the term itself.  As for the importance of input bias current, if the resistors in your design are sufficiently large, then this bias current can contribute a non-negligible and undesired voltage drop across these components.  As Kai has pointed out, this is rarely an issue, especially when your typical bias current is 5pA as for this part.

Here are some more e2e threads on this topic:

• https://e2e.ti.com/support/amplifiers/f/14/t/356932?Input-Bias-Current-vs-Input-Impedance
• https://e2e.ti.com/support/amplifiers/f/14/t/466732?OPA333-input-impedance
• https://e2e.ti.com/support/amplifiers/f/14/t/656428?OPA330-Input-Impedance

Here's a good resource by Bruce Trump that goes into further detail.

Regards,

Daniel

Hi Daniel and Kai,

Truly appreciate for your advice.

To answer Kai's question, my resonant microphone is basically the microphone having a resonant filter integrated. For example, the output of 2kHz resonant microphone is filtered the sound signal at 2kHz.

The output impedance of microphone is about 10^9 Ohm so I assume TΩ input resistance of preamp is sufficient.

One concern ,though, is the output voltage level from the microphone is quite small which is 0.2~4mV. Probably, minimizing the noise is significant and I need a DC bias voltage on the input signal to pre amp.  

Attached is brief block diagram for you to understand the system more clearly.( This is not a full block diagram but only microphone to speaker blocks)

It would be appreciated if you have a better recommendation for pre-amp, summing op amp, and audio amp on the above block diagram.

Thanks and regards,

Jaehoon

Hi Jaehoon,

you wrote that a DC bias is necessary. Is it for powering the microphone or to just allow AC signal handling? In the first case, a datasheet of microphone would be needed. In the second case, I would use a bipolar supply voltage, for instance +/-2.5V.

I don't think that noise will be too much of an issue because you can use bandpass filtering after the gain stage to remove low frequency and high frequency noise.

Kai

Hi Jaehoon,

Apologies, but I missed your request for some part recommendations.

If you would like to lower your noise, consider the TLV6742.  However, as Kai has alluded to, signal filtering may sufficient on its own.

Do you have DC offset concerns?  If you have a very large gain, then using a part with a small input bias current and small current noise might be worth your consideration.

If you still need help selecting parts, please let me know your concerns and I can make some recommendations.

Regards,

Daniel