Troubleshooting Bias voltages on AD5941

Hi,

In the example,

1)

Bias volts = VBIAS - VZERO

VZERO = 1.1V

Bias volts =0

Hence, VBIAS = 1.1V

2)

Voltage of CE0 with respect to ground = VBIAS (If SinAmplitudeWord = 0)

Voltage of SE0 with respect to ground = VZERO

3)

If SinAmplitudeWord is not 0,

    Sine voltage waveform can be observed when monitored across CE0 and SE0 (VCE0 - VSE0).

4)

In EIS example,

if(lploop_cfg.LpDacCfg.DacData12Bit>lploop_cfg.LpDacCfg.DacData6Bit*64)
lploop_cfg.LpDacCfg.DacData12Bit--;

above lines correct the DAC output value after removing the loading effect as mentioned in datasheet:

In AD5940_Impedance example,

this loading effect compensation is not done as VZERO is fixed to middle scale value.

The below lines:

if(AppIMPCfg.BiasVolt<-1100.0f) AppIMPCfg.BiasVolt = -1100.0f + DAC12BITVOLT_1LSB;
if(AppIMPCfg.BiasVolt> 1100.0f) AppIMPCfg.BiasVolt = 1100.0f - DAC12BITVOLT_1LSB;

ensure that VBIAS <=2.2V.

Hi Akila,

Thanks for the response. 

Just to be clear,

1. So I can have a relative bias that is less than zero, correct? (VBIAS < VZERO)? So going back to that example, if I wanted SE0 to have a relative bias of -100 mV, I can set VBIAS to 1.0V and VZERO to 1.1V? 

2. If SinAmplitudeWord is not 0 (that is, if I have a set amplitude for the sine wave and it is active), I should see VBIAS if I look at CE0-SE0? Or is it VBIAS - VZERO? If it's VBIAS - VZERO, is that the same DC bias that SE0 is then experiencing? Because I verify it using the VBIAS and VZERO PINS, but can't seem to get similar results when looking at SE0. 

3. Is Voltage of SE0 with respect to ground = VZERO still valid if SinAmplitudeWord is not 0? 

Hi,

1)

VBias can be less than Vzero.

 To get VBias - Vzero = -100mV,

set AppIMPCfg.BiasVolt = -100.0f

2)

VCE0 -VSE0 = VBIAS -VZERO

3) Since in the above impedance example, SE0 is connected to input of TIA, and other input f TIA is connected to VZERO,

voltage at SE0 w.r.t. GND is always VZERO irrespective of value of SinAmplitudeWord.

Hi Akila, 

I set Vbias to 100 mV, and I measure that the VSEO w.r.t VCEO is -100 mV. This is correct?

Even though I'm measuring -100 mV for VSEO - VCEO (VZERO - VBIAS), does this mean that the SE0 voltage is still experiencing a positive voltage of +100 mV?  Because if VSE0 is the working electrode and is set as VZERO, doesn't that mean that in the case of VBIAS = 100 mV, the reference (VBIAS) will always have a higher potential and therefore VSE0 will experience -100 mV?  

So, if I want the working electrode (VSE0) to experience a bias of +100 mV, doesn't that mean VBIAS needs to be -100 mV so that VSE0 relative to the RE0 and CE0 will be +100 mV? Is there any way to turn on the DC bias first before sending an AC waveform through CE0? I did so by turning off the WG before I initialized everything else, delayed to a set time and then turned the WG on. This seemed to work to look at solely the DC bias without a superimposed waveform, but wanted to ask if there was a proper way to look at it that I was missing.

For reference, here's how I have it set up in code: 

Hi,

Yes. If Vbias is set to 100 mV, VSEO w.r.t VCEO is -100 mV.

If SE0 is connected to input of TIA, and other input of TIA is connected to VZERO,

voltage at SE0 w.r.t. GND is always VZERO.

VCE0 = VZERO + 100mV.

VSE0 = VZERO

Yes. You may turn of WG if only DC bias is to be applied.

You may measure the DC bias output by measuring voltage across CE0 and SE0.

RE0 need not be tied to CE0 for this measurement.

Hi, 

I disagree with RE0 not being tied to CE0. I find that for a default 1.1V measurement, CE0 measures 1.45V and RE0 measures 0.45 V. Tying CE0 and RE0 together gets 1.1V, and measuring SE0 w.r.t CE0&RE0 gets you a bias voltage of 0.0 (as expected). I'm not sure if that's intended but that's the result I've gotten across multiple breakout boards and sensors.  

Following up on this: I verified that the 2.5V, 1.8V references are valid. The electrode setup is the same as the previous comments. I've also attached other lines of code for reference in how I do the bias: 

Hi kbaudue ,

I've responded to your other post regarding CE, RE, and SE connections.

regards,

Mark