Update AMDS sensor card docs to provide an expression for the sensed quantity in terms of the ADC voltage

[elsevers]

Abstract

We want each of the AMDS sensor card docs to provide an expression for the sensed quantity (i.e., current in amps for the current card) in terms of the ADC voltage to make it easy for users to read from the AMDS.

Context

AMDC-Firmware PR #449 is adding a new function to help users interpret their AMDS ADC data:

https://github.com/Severson-Group/AMDC-Firmware/blob/6083845366af2af6a3a036e42e759e632418f439/sdk/app_cpu1/common/drv/amds.c#L124-L128

int amds_get_calibrated_data(double converted_voltage, double offset, double gain, double *out)
{
    *out = (converted_voltage - offset) * gain;
    return SUCCESS;
}

We want to make it easy for users to know what to use for offset and gain for each of the AMDS cards.

1. Current Card

The AMDS current card currently does the opposite of what this issue is requesting (it provides the ADC terminal voltage in terms of the sensed quantity).

We can solve these expressions for Iprimary to get the sensed current.

2. Low Voltage Card

The low voltage card does not provide any data, but we can see from its schematic that VADC = 0.1*(VMEAS) + 2.048V:

3. High Voltage Card

The high voltage card provides an expression that can be solved for this:

Approach

Add to each of the AMDS card docs a section called ### Relationship Between Input and ADC voltage that provides an expression in the form of INPUT = (VADC - OFFSET) x GAIN

For example, in the current measurement REV B, we would have:
$I_{\rm PRIMARY} = (V_{\rm ADC} - 2.4922) \times 29.4118 {\rm [ A ]} $

First do this:

As a first step, add a comment to this issue with the INPUT = (VADC - OFFSET) x GAIN for each of the three cards based on the default component values

[Daehoon-Sung]

Current Card

For REV B,

$$ I_{\text{PRIMARY}} = (V_{\text{ADC}}-2.4922) \times 29.4118 [A] $$

For REV C,

$$ I_{\text{PRIMARY}} = (V_{\text{ADC}}-2.5126) \times 29.4118 [A] $$

---

Low voltage Card

$$ V_{\text{meas}} = V_{\text{IN+}} - V_{\text{IN-}} = (V_{\text{ADC}} - 2.048) \times 10 [V] $$

---

High Voltage Card

• The expression below from AMDC Platform article is wrong.

• Correct relationship between $V_{\text{IN}}$ and $V_{\text{ADC}}$ cannot be obtained using this one.
• $R_{IN}$ in the equation is unclear. I tried $R_{IN}$ = $R_{9}$ and $R_{IN}$ = $R_{9}$ + $R_{10}$ (since it is series circuit), but both show big difference in gain from what Ryan got from experiment. I attached the input part of the high voltage card as below.

• I derived the equation from the base. The results coincides with what I and Ryan got from the experiments. I attached this as pdf HighVolt_Calculation.pdf hoping to be helpful for other students. NOTE: There is typo at the last of the HighVolt_Calculation $K_N = N_P/N_S = 2.5$. (The calculation is correct)
• Final formula is,

$$ V_{\text{IN}} = (V_{\text{ADC}} - 2.5031) \times 256.0223 [V] $$

• In fact, using the formula from my current sensor calibration report which is:

$$ V_{\text{out}} = \left[\frac{R_{4} R_{6} (R_{4} + R_{5})}{(R_{4} R_{6} + R_{4} R_{8} + R_{6} R_{8})(R_{4} + R_{5}) - R_{5} R_{6} R_{8}}\right] V_{\text{ref}} + \left[\frac{R_{4} R_{5} R_{6} R_{8}}{(R_{4} R_{6} + R_{4} R_{8} + R_{6} R_{8})(R_{4} + R_{5}) - R_{5} R_{6} R_{8}}\right] I_{\text{s}} $$

and plug in the transformer turns ratio with input resistor to get relationship between secondary current and input voltage as below,

$$ I_{\text{s}} = \left[\frac{K_{N}}{(R_{9}+R_{10})}\right] V_{\text{IN}} $$

The final formula can be obtained and it is same as I have calculated.

Thank you, Professor @elsevers .

[elsevers]

Thanks @Daehoon-Sung!

[Known4225]

The offset value for the low voltage card should be 0 instead of 2.048

This is because the ADC reports the measured voltage relative to the 2.048V reference

[Daehoon-Sung]

For the high voltage card,

Rev A

$R_4 = 10 k\ohm$, $R_5 = 390\ohm$, $R_6 = 8.45 k\ohm$, $R_8 = 4.64 k\ohm$,
$R_9 = R_{10} = 25 k\ohm$, $K_N = N_P/N_S = 2.5$, $V_{REF} = 5 V$

$$ V_{\text{IN}} = (V_{\text{ADC}} - 2.5054) \times 229.170 [V] $$

Rev B, Rev C

$R_4 = 10 k\ohm$, $R_5 = 348\ohm$, $R_6 = 8.45 k\ohm$, $R_8 = 4.64 k\ohm$,
$R_9 = R_{10} = 25 k\ohm$, $K_N = N_P/N_S = 2.5$, $V_{REF} = 5 V$

$$ V_{\text{IN}} = (V_{\text{ADC}} - 2.5031) \times 256.022 [V] $$

[Daehoon-Sung]

Hello Professor @elsevers Dr @noguchi-takahiro . I think we need to huddle up together to review this one.
Do you prefer to review this during the office hour or do you want to setup a specific time for this?