TL441: Perform an impedance measurement

Hi Stefano,

The TL441-EP is a somewhat obscure LOG amplifier and we don't have much information about it. Nevertheless, we will attempt to assist you with your questions.

Can you tell me which configuration, or figure, you are making reference to in the TL441-EP datasheet? Can you describe how you are thinking about configuring it for an impedance measurement? Once we have this information we can consider your questions about the A and B inputs, and gain.

Regards, Thomas
Precision Amplifiers

Hi Thomas,

at page page 13 figure 16 of the data-sheet you can use it to perform a Multiplication or Division. As stated in note A you can perform a division between two measurement simply swapping Z and Z! (i mean not Z). If input A is a voltage measurement and input B is a current measurement dividing A and B you perform an impedance measurement.

thanks in advance 

Stefano

Hello Stefano,

Since we have nothing but the TL441 datasheet to reference this is going to be a learning experience for both of us! I will attempt to answer your questions, but some discussion may be needed to get to the answers.

1) what are A2 and B2 and why i need to input the signals both in A1 and in A2?

Refering to the datasheet Figure 16. Multiplication or Division circuit there are only 2 inputs to the function block, Input A and Input B . Therefore, to utilize the TL441 in division mode the external input voltages will interface with Input A and Input B, which are also the A1 and B1 pin descriptors. The A2 and B2 inputs are not driven directly by an external source, but via the inputs amplifiers connected to those inputs.

2) and in B1 and B2 being the two in A2 and B2 amplified by the two op-amp?

I think this covered by my answer to the first question.

3) How much is the gain i have to set for the op amp in A2 and B2?

The datasheet provides no clue about the gain of the op amp amplifiers that precede the A2 and B2 inputs. However, the equation for the division output W is stated as:

So it appears that the divide circuit output level W has an exponential relationship that is a function of "a" where "a" is the base of the log. This seems to be supported by Figure 18. Raising a Fixed Number to a Variable Power, where "a" is described as "Exponential to any base: W = a."

My thought is "a" is the gain. If that is the case setting "a" to 10 V/V (20 dB) would result in an output voltage level W that is equal to 10, raised to an exponent where the log is base 10.

I m not sure if this correct; there's just no information to go on. You will note that in some of the other TL441 application circuits shown that the gain can be potentially set high so I may not be too far off!

Regards, Thomas

Precision Amplifiers Applications Engineering 

Hi Stefano,

Although the device does not reside within the Preicison Amplifiers product family, TI has a current shunt monitor that provides instantaneous power monitoring capability. It provides a straightforward power measurment solution. It is the INA223; you can find the datasheet here:

www.ti.com/.../ina223.pdf

That might provide a good alternative to the TL441 log amplifier approach.

Regards, Thomas
Precision Amplifiers Applications Engineering