PmodIA Extension

[Nianyu Jiang]

Hello guys,

I am trying to extend PmodIA from 2 electrodes to 4 electrodes (for better accuracy). I got some paper talking about how to do that directly from AD5933 chip, but I see there is already some electrical components integrated on the PmodIA board, so I am trying to figure out the circuit.

The attachment is PmodIA scheme and I highlight the part which I am trying to understand.

1. I believe this is high pass filter to move the offset.

2. I believe this is Op-Amp, but why do we need this?

3. I think this is a switch or multiplexer, but why do we need this here?

4. Same as part 2.

I would be really appreciate if someone can give me a brief answer or forward me some related material.

Thanks a lot!

Nianyu Jiang

[JColvin]

Hi @Nianyu Jiang,

1. I believe this is high pass filter to move the offset. -- That is correct.

2. I believe this is Op-Amp, but why do we need this? -- This op-amp is arranged as a voltage follower, to help further stabilize the signal.

3. I think this is a switch or multiplexer, but why do we need this here? & 4. Same as part 2. --  This is part of the feedback amplifier that goes from the received signal (after passing through the device under test) to internal ADC, detailed further on page 15 of the AD5933 datasheet and shown again in Figure 24 on page 18. As this user mentions in their post, driving SEL to ground sets the feedback resistor (what the mux/switch is selecting) to 100 kOhms and setting SEL to Vdd sets the feedback resistor to 20 Ohms. The choice in the feedback resistors (as well as the gain factor and output excitation voltage range selected in the Control Register) affects the result which needs to stay within the internal ADC input range of 0V to Vdd. To figure out if the gain through the system is within the ADC voltage input range, follow the calculation presented on page 18:

Quote

Output Excitation voltage * Gain Setting Feedback Resistor * PGA Gain / Unknown Impedance

You'll note that this inherently limits what can be reliably measured with regards to the unknown impedance depending on what settings you have chosen, which is also noted in the user post I linked to earlier.

Let me know if you have any questions.

Thanks,
JColvin

[Nianyu Jiang]

4 hours ago, JColvin said:

Hi @Nianyu Jiang,

1. I believe this is high pass filter to move the offset. -- That is correct.

2. I believe this is Op-Amp, but why do we need this? -- This op-amp is arranged as a voltage follower, to help further stabilize the signal.

3. I think this is a switch or multiplexer, but why do we need this here? & 4. Same as part 2. --  This is part of the feedback amplifier that goes from the received signal (after passing through the device under test) to internal ADC, detailed further on page 15 of the AD5933 datasheet and shown again in Figure 24 on page 18. As this user mentions in their post, driving SEL to ground sets the feedback resistor (what the mux/switch is selecting) to 100 kOhms and setting SEL to Vdd sets the feedback resistor to 20 Ohms. The choice in the feedback resistors (as well as the gain factor and output excitation voltage range selected in the Control Register) affects the result which needs to stay within the internal ADC input range of 0V to Vdd. To figure out if the gain through the system is within the ADC voltage input range, follow the calculation presented on page 18:

You'll note that this inherently limits what can be reliably measured with regards to the unknown impedance depending on what settings you have chosen, which is also noted in the user post I linked to earlier.

Let me know if you have any questions.

Thanks,
JColvin

Hello Colvin,

Thanks for the answer.

I understand your answer for 1 & 2.

For the 3 & 4, since we need the ADC input signal stay in the range of 0 - Vdd, we need to use the mux (part 3) to select the resistor to choose the gain (5 or 1), is that right?

I will read more on datasheet and the post you forward to me to understand the equation.

Thanks a lot!

Nianyu Jiang

[JColvin]

Hi @Nianyu Jiang,

Not quite; the mux selects external feedback resistor used. The programmable gain (1x or 5x) is controlled by bit D8 in the Control register (address 0x80) within the embedded AD5933. Similarly, the output voltage range (whether 2V, 1V, 400mV, or 200mV peak-to-peak) is also controlled by bits D10 and D9 in the same Control register address.

Can you also link to the paper you saw that talks about expanding the number of electrodes? I'm having difficulty finding it.

Let me know if you have any further questions.

Thanks,
JColvin

 

[Nianyu Jiang]

7 hours ago, JColvin said:

Hi @Nianyu Jiang,

Not quite; the mux selects external feedback resistor used. The programmable gain (1x or 5x) is controlled by bit D8 in the Control register (address 0x80) within the embedded AD5933. Similarly, the output voltage range (whether 2V, 1V, 400mV, or 200mV peak-to-peak) is also controlled by bits D10 and D9 in the same Control register address.

Can you also link to the paper you saw that talks about expanding the number of electrodes? I'm having difficulty finding it.

Let me know if you have any further questions.

Thanks,
JColvin

 

https://www.researchgate.net/publication/236037769_A_four-electrode_low_frequency_impedance_spectroscopy_measurement_system_using_the_AD5933_measurement_chipt

this is the paper I am talking about.

Thanks for the further explaination, I start understanding the working principle and trying to combine everything.

Will go back to you once I have more question.

Nianyu Jiang