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ADS8325: current of REF and DC characteristic by DCLOCK

cafain
Mastermind 6615 points
Part Number: ADS8325
Other Parts Discussed in Thread: REF6025, ADS8900B

Hi,

When changing DCLOCK speed and timing, I want to check the DC characteristics and the relation of the input current of REF
I mailed it.

Please respond to this question.

1.When delaying only f_CLK with the specification condition (VDD=+5V) on the Datasheet (for example, 2.4MHz -> 24kHz)
  Is there a change in DC characteristics?
  If there is, can you give me reference data?

  In other words
  ELECTRICAL CHARACTERISTICS conditions are fSAMPLE=100kHz, fCLK=2.4MHz it is specified by.
  For example, when using fSAMPLE=1kHz and fCLK=24kHz,
  are there changes in ELECTRICAL CHARACTERISTICS parameters (specifications)?

2.In the data sheet, it is described as "reference input current depends on the conversion rate of the ADS8325."
  When changing the frequency and timing of DCLOCK as shown in (a), (b), (c) of the waveform diagram,
  Is there a change in the current flowing through the REF pin?
    (When REF terminal is attached with 47 μF capacitor)

best regards

  • 0
    TI__Expert 3775 points
    Hi Cafain

    1) DC characteristics of the device will not have a major change with varying throughput. However, due to the change in rate of switching activity, the signal chain components such as input driver and reference driver may perform better at lower speeds (if not designed for full throughput). The ADC's DC characteristics will be consistent over the specified throughput range.

    2) The reference current is a switching current. For a given input voltage fixed amount of charge is drawn from reference of the ADC. Over how much period this charge is drawn depends on the throughput of the device.
    For example, assume the device draws 100 pico-coulombs charge from reference for a given input voltage. This 100 pico-coulombs charge can be drawn in 10us (100kHz) or in 100us (10kHz). Hence the current drawn from reference at 100kHz throughput is 10 times higher than the current at 10kHz throughput.
    Charge movement remains the same, only the time over which the charge is delivered changes and hence reference current scales proportionally with throughput.

    Regards,
    Rahul
  • 0 in reply to RahulVK
    Mastermind 6615 points

    Hi, Rahul san

    I appreciate your early support.

     2) Please let me check again for the answer.

    I understood that the reference power supply current changes with the conversion rate.

    How much is the changing reference current value?
    Is it 1mA (typ) and 1.5mA (max) of "Reference input current" written in Datasheet page4?

    best regard

    cafain
    　　

  • 0 in reply to cafain
    TI__Expert 3775 points
    Hi Cafain-san,

    The 1mA typical is the reference current at room temperature i.e. 25°C. Over the temperature range of -40°C to +85°C, the reference current will be maximum 1.5mA.

    Which reference voltage IC are you using? Have you had a chance to look at REF6025? REF6025 (REF60xx family) is a voltage reference with integrated buffer capable of driving SAR-ADCs.

    Regards,
    Rahul
  • 0 in reply to RahulVK
    Mastermind 6615 points

    Rahul san

    I will also consider REF 6025.
    I appreciate the detailed support.

    Regards,

    Cafain

  • 0 in reply to cafain
    Mastermind 6615 points

    Hi, rahul san

    In addition please let me question the following 3 points.

    (1) When timing from CS assertion to sampling start is changed from (b) to (c)
        is there a difference in the reference current ?

    (2) Figure37 shows that the reference current is 80uA when VDD=5V and Sample Rate =10kHz,
        but how much is this (error) as a variation of this value ?

    (3) Under the condition of (2) above and with a 47μF capacitor attached to the REF terminal,
        the ripple ΔV_Ref of the reference voltage during sampling is

       ΔV_Ref = (I_Ref / C_Ref) × Δt
       = (80 uA / 47 uF) * 50 usec (*)
                 ≈ 80 uV

       Could it be safe to think that? (Calculated with * CLK duty as 50%)

    best regards

    cafain

  • 0 in reply to cafain
    TI__Expert 3775 points
    Hi Cafain-san,

    (1) The charge drawn from reference, for a given input voltage, is same for any conversion rate. When going from (b) to (c), you are changing the conversion rate i.e. the time over which this charge is delivered. Hence the average reference current will scale accordingly.

    (2) If I understood your question correctly, you want to know at what clock rate is the ADC operating when it draws 80uA average current from reference. The reference current in this figure is measured when the converter is clocked at 2.4MHz clock. If the conversion clock rate is charged, average reference current will change accordingly (lower reference current for lower conversion clock)

    (3) The ripple on the reference pin of an ADC is a function of sampling rate and conversion clock frequency. For a detailed discussion on this topic you could refer to the "Reference Buffer Module" section of ADS8900B www.ti.com/.../ads8902b.pdf datasheet. However, your equation is a simplification of this model. The reference driver would be providing the average reference current I_ref and hence the voltage ripple would be a function of the output impedance of the reference driver as well.

    Regards,
    Rahul
  • 0 in reply to RahulVK
    Mastermind 6615 points

    Rahul san

    I appreciate the quick response.

    I will comment on the question of (2).
    It is the sampling rate of , VDD = 5V and Sample Rate = 10kHz (FCLK = 240 kHz).
    I would like to know the reference current value of the VREF pin in this case and its current value error (variation).

    I am sorry to ask you many times.

    Best regards

  • 0 in reply to cafain
    TI__Expert 3775 points
    Hi Cafain-san,

    In Figure 37, FCLK = 2.4-MHz for all sample rates.
    Iref is a typical parameter in the data-sheet and hence a typical curve has been provided at room temperature.

    Importantly, the reference current will change depending on the output code of the ADC. This parameter will also change with temperature, reference voltage and conversion rate of the ADC. Hence it is difficult to estimate the spread of reference current given all these factors.

    Regards,
    Rahul