units of calibration coefficients?

[Hylocereus]

Hey pitaya users,

i am about to calibrate my pitaya and i don't fully understand the calibration vector you get with the "calib -r -v" command.
I understand the offset calibration as it is described in the user manual but what about the "full scale coefficients"?

FE_CH1_FS_G_HI = 45870551 # Front-End Channel1 full scale coefficient for High gain (LV) jumper configuration.
FE_CH2_FS_G_HI = 45870551 # Front-End Channel2 full scale coefficient for High gain (LV) jumper configuration.

What are the units of these numbers? I expect something like "samples per volts" but the number is much larger.
So if you acquire a measured number of samples with the "acquire" command, how do you compute a measured voltage out of it with this coefficient and the offset?

I would really appreciate any help on this.
Regards

[Nils Roos]

These are the sections of code that transform samples to voltages by way of the calibration values:

Code: Select all

float sample_to_voltage(int sample) {
	float voltage, ch1_max_adc_v;
	uint32_t fe_fsg1;

	fe_fsg1 = (curr_params[GAIN_CH1].value == 0) ? rp_calib_params->fe_ch1_fs_g_hi
	                                             : rp_calib_params->fe_ch1_fs_g_lo;

	ch1_max_adc_v = osc_fpga_calc_adc_max_v(fe_fsg1, (int)curr_params[PRB_ATT_CH1].value);

	voltage = osc_fpga_cnv_cnt_to_v(sample, ch1_max_adc_v, rp_calib_params->fe_ch1_dc_offs, 0);

	return voltage;
}

/*----------------------------------------------------------------------------*/
/**
 * @brief Converts ADC counts to voltage [V]
 *
 * Function is used to publish captured signal data to external world in user units.
 * Calculation is based on maximal voltage, which can be applied on ADC inputs and
 * calibrated and user defined DC offsets.
 *
 * @param[in] cnts           Captured Signal Value, expressed in ADC counts
 * @param[in] adc_max_v      Maximal ADC voltage, specified in [V]
 * @param[in] calib_dc_off   Calibrated DC offset, specified in ADC counts
 * @param[in] user_dc_off    User specified DC offset, specified in [V]
 * @retval    float          Signal Value, expressed in user units [V]
 */
float osc_fpga_cnv_cnt_to_v(int cnts, float adc_max_v,
                            int calib_dc_off, float user_dc_off)
{
    int m;
    float ret_val;

    /* check sign */
    if(cnts & (1<<(c_osc_fpga_adc_bits-1))) {
        /* negative number */
        m = -1 *((cnts ^ ((1<<c_osc_fpga_adc_bits)-1)) + 1);
    } else {
        /* positive number */
        m = cnts;
    }

    /* adopt ADC count with calibrated DC offset */
    m += calib_dc_off;

    /* map ADC counts into user units */
    if(m < (-1 * (1<<(c_osc_fpga_adc_bits-1))))
        m = (-1 * (1<<(c_osc_fpga_adc_bits-1)));
    else if(m > (1<<(c_osc_fpga_adc_bits-1)))
        m =  (1<<(c_osc_fpga_adc_bits-1));

    ret_val =  (m * adc_max_v / 
                (float)(1<<(c_osc_fpga_adc_bits-1)));

    /* and adopt the calculation with user specified DC offset */
    ret_val += user_dc_off;

    return ret_val;
}

/*----------------------------------------------------------------------------*/
/**
 * @brief Calculates maximum [V] in respect to set & calibration parameters
 *
 * Function is used to calculate the maximal voltage which can be applied on ADC inputs.
 * This calculation is based on calibrated Front End Full Scale Gain setting and configured
 * Probe Attenuation.
 *
 * @param[in] fe_gain_fs     Front End Full Scale Gain
 * @param[in] probe_att      Probe attenuation
 * @retval    float          Maximum voltage, expressed in [V]
 */
float osc_fpga_calc_adc_max_v(uint32_t fe_gain_fs, int probe_att)
{
    float max_adc_v;
    int probe_att_fact = (probe_att > 0) ? 10 : 1;

    max_adc_v = 
        fe_gain_fs/(float)((uint64_t)1<<32) * 100 * (probe_att_fact);

    return max_adc_v;
}

(pieced together from the scope app's worker.c and fpga.c , see there for the missing constants, types and parameters)

The unit of the calibration value 'full scale coefficient' is V/cnt , but there are some constants factored into it. This is why its magnitude seems kind of arbitrary.

[edgo]

viewtopic.php?f=14&t=510

You may find my calibration utility helpful