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# AD5940: SWV sample delay calculation?

Category: Hardware

I have two SWV settings (low freq) and (high freq). I always have had a hard time understanding how to correctly calculate when to sample the current using the sample delay variable. The repo states the equation is < (1/Frequency)/2 - 0.2*/.

If Frequency = 25 hz then sample delay = (1/25)/2 - 0.2 =  -0.18 ms?

If Frequency = 300 hz then sample delay =  -0.19 ms?

Generally does this mean we do not change this variable? My parameter at low freq 25 hz is set to 2.0f (ms) and seemingly works ok.

Here is the code:

pRampCfg->SeqStartAddr = 0x10; /* leave 16 commands for LFOSC calibration. */

pRampCfg->MaxSeqLen = 1024 - 0x10; /* 4kB/4 = 1024 *
pRampCfg->RcalVal = 10000.0; /* 10kOhm RCAL*/
pRampCfg->ADCRefVolt = 1820.0f; /* The real ADC reference voltage. Measure it from capacitor C12 with DMM. */
pRampCfg->FifoThresh = 1; /* Maximum value is 2kB/4-1 = 512-1. Set it to higher value to save power. original 480 */
pRampCfg->SysClkFreq = 16000000.0f; /* System clock is 16MHz by default */
pRampCfg->LFOSCClkFreq = LFOSCFreq; /* LFOSC frequency */

/* Step 2:Configure square wave signal parameters */ LOW FREQUENCY
pRampCfg->RampStartVolt = 0.0f; /* -1.4V */
pRampCfg->RampPeakVolt = 600.0f; /* -0.1V */
pRampCfg->VzeroStart = 1300.0f; /* 1.3V */
pRampCfg->VzeroPeak = 1300.0f; /* 1.3V */
pRampCfg->Frequency = 25; /* Frequency of square wave in Hz */
pRampCfg->SqrWvAmplitude = 50; /* Amplitude of square wave in mV */
pRampCfg->SqrWvRampIncrement = 4; /* Increment in mV*/
pRampCfg->SampleDelay = 2.0f; /* 10ms. Time between update DAC and ADC sample. Unit is ms. */
pRampCfg->LPTIARtiaSel = LPTIARTIA_1K; /* Maximum current decides RTIA value */
pRampCfg->bRampOneDir = bTRUE;

===========================================

/* Step 2:Configure square wave signal parameters */ HIGH FREQUENCY
pRampCfg->RampStartVolt = 0.0f; /* -1.4V */
pRampCfg->RampPeakVolt = 600.0f; /* -0.1V */
pRampCfg->VzeroStart = 1300.0f; /* 1.3V */
pRampCfg->VzeroPeak = 1300.0f; /* 1.3V */
pRampCfg->Frequency = 300; /* Frequency of square wave in Hz */
pRampCfg->SqrWvAmplitude = 25; /* Amplitude of square wave in mV */
pRampCfg->SqrWvRampIncrement = 5; /* Increment in mV*/
pRampCfg->SampleDelay = 0.2f; /* 10ms. Time between update DAC and ADC sample. Unit is ms. */
pRampCfg->LPTIARtiaSel = LPTIARTIA_1K; /* Maximum current decides RTIA value */
pRampCfg->bRampOneDir = bTRUE;//bTRUE; /* Only Measure Ramp in One Direction leave as true - omeed */
pRampCfg->floor = 0;

Added the part number to the product page
[edited by: JEstayo at 1:42 AM (GMT -5) on 4 Mar 2024]
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• Hi,

Only relation between frequency (f) and sample delay (d) is below:

(2/f)  > d

Sample delay is the time taken to do measurement of one sample.

Minimum value of sample delay is chosen depending on value of WaitClks in AppSWVSeqADCCtrlGen() sequence.

Typically,

for low frequencies, sample delay must be greater than 1ms.

for high frequencies, sample delay must be greater than 0.2ms.

• Hi,

Only relation between frequency (f) and sample delay (d) is below:

(2/f)  > d

Sample delay is the time taken to do measurement of one sample.

Minimum value of sample delay is chosen depending on value of WaitClks in AppSWVSeqADCCtrlGen() sequence.

Typically,

for low frequencies, sample delay must be greater than 1ms.

for high frequencies, sample delay must be greater than 0.2ms.

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