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MSP430F67xx SD24 input impedance..

Ernest Cho
Expert 3341 points

Other Parts Discussed in Thread: MSP430I2040

Hello Champs,

I think this customer schematic would have an issue with source impedance.  And I recommended to reduce at least 10x lower than SD24 input impedance.

(Their SD24 + input has series of 100Kohm+100K(NTC) between 1.225V(VREF_external) and GND.
SD24- input directly connected with GND.)

(related posts I found :  )

But customer want to have vivid documents or explain with equation.

I didn't found any documents guidance, 
So I thought I could explain it to customer with combined impedance of internal and external resisters.

And I made excel equations as attached to calculate several V, I to explain the voltage drop of SD24 + input,
but it showed different results vs. measured results.

In short, I assumed the SD24 turn on circuit results the voltage drop of SDxP0 and calculated data and measured data will matched.
(I also assumed SD24 start of conversion will turn on the 200k-ohm input impedance.)
But the measured data is around 10mV, but calculated value is 122.5mV.

I don't know which one is wrong. Could you someone advise me?
It would also appreciate to suggest any SD24 application guidance regarding impedance.

<Calculation - refer excel attachment>

V_REF 1.225 V value
R46 100 k-ohm value
RT1 100 k-ohm value
internal R 200 k-ohm value
V_R46(off) 0.6125 V result
V_RT1(off) 0.6125 V result
I_R46(off) 6.125 uA result
I_RT1(off) 6.125 uA result
RT1 // R_in 66.666667 k-ohm result
V_R46(on) 0.735 V result
V_RT1(on) 0.49 V result
I_R46(on) 7.35 uA result
I_RT1(on) 4.9 uA result
I_R_internal 2.45 uA result
V_diff_on/off 122.5 mV result

< schematic > 

< Scope result >

<Excel Calculation>
SD24 impedance calculation_2016_1018.xlsx

Thank you.

  • 0
    TI__Expert 3341 points
    I have one more question.
    Customer also has issue on the spike wave form at the Start of Conversion time.
    They reduced it using below two solutions :
    1. settling time calculation and adjust fM which noted at User's guide 29.13.3 "Analog input Characteristics".
    2. External RC filter.

    Could you please advise for the external RC filter value?
    I mean, assuming 2 or 3*fM as cut off frequency, should we consider internal 1K ohm and Cs with external user R and C?
  • 0 in reply to Ernest Cho
    TI__Guru 73461 points
    Ernest,
    I agree with what you have suggested. Please note that the input impedance AND the differential input impedance are applicable when the SD24 modulator is OFF and ON (table 5-21, datasheet) . Also, these values are typical with 300 kohms provided as the minimum for the differential. I do not think it is wise to assume that the impedance will remain constant between the on and off states. It may be possible that the difference that you are seeing is simply the difference in impedance when the IO are on and off.

    I will try and setup an experiment where I source a voltage through 50K, 10k,1K, and 100 ohms and try to recreate the same waveforms. I will give you an update on Wednesday.

    Thanks,
    Chris
  • 0 in reply to Chris Sterzik
    TI__Guru 73461 points
    Ernest,
    I setup the test so that I supplied 1.225V to the reference and 0.66V to the input + pin via a resistor. The input – pin was shorted to GND. I failed to run the 50K test case but this is what I got for the following resistor values:

    Source (V) R (ohms) Von (V) Voff (V)
    0.66 100000 0.518 0.444
    0.66 10000 0.64 0.631
    0.66 1000 0.659 0.657


    As you have already described, lowering the source impedance is important. I did not see the noise spike that you described, but this may be related to the target board and test setup. Please let me know if you have additional questions.

    Regards,
    Chris
  • 0 in reply to Chris Sterzik
    TI__Expert 3341 points

    Chris,

    Thanks for your update.
    Due to the customer time line, I made report and reported it to customer.
    In the report, I varified why spike and drop are happened.
    We can calculate the V, I and R only using Datasheet and User's guide, and those explain the voltage drop for certain external R value.

    In the customer board where they used 100K and 100K divided R between 1.225V and GND,
    the spike is happening after SD24 start conversion, and drop is happening after modulator off-ed.
    At the start conversion time, the SD24 internal R is around 200K at the single ended mode.
    And at the modulator off-ed time, the SD24 internal can measured by current leakage graph in the datasheet.

    I don't know if I can attach the report here. The report will help why the issue happened. I will send email to you.

    One more question,
    From the customer measured wave form, there exist 3 state in the single conversion of SD24 modulation.
    1. Conversion,
    2. SD24 Modulator off,
    3. SD24 module disable

    Is there any time delay data spec of 2->3 state change?

    Regard,
    Ernest

  • 0 in reply to Chris Sterzik
    TI__Guru 73461 points

    Ernest,

    Please disregard this information for the context of this discussion.  This data is for the MSP430I2040 which has the SD24 and not the SD24_B which is in the MSP430F67xx.  Also, the note regarding the differential and input impedance found in the I2040 is not applicable to the MSP430F67xx.

    Chris

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