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TMS570LS3137: ADC input currrent for pin AD1IN[4]( U18) for TMS570LS3137 microcontroller

Part Number: TMS570LS3137

we are using TMS570LS3137 microcontroller in our design. we need to understand Figure 7-1. MibADC Input Equivalent Circuit in more detail.

we need to calculate input ADC current. our Analog input voltage is given to pin AD1IN[4]( U18).

while calculating AD1IN[4]( U18) pin input current do we need to consider only on state bias current (IAOSB1) or should we also add leakage current of other channels also? In figure on state bias current is shown as addition of other channel leakage current. please explain this.

what is ther roll of Rsamp and Csamp? as per my understanding after Csamp there should be OPAM hence it should have high impedence. so maximum voltage Csamp can attain is equal to Cmux. please help us to understand this more.

sampling frequency in our case is 1KHz.

  • Hi Lidong,

    If a shared channel is being converted by both ADC converters at the same time, the on-state leakage is equal to IAOSB1 + IAOSB2.

    I will come back to you tomorrow.

  • There are two CMOS switches in the path between the ADINx pin and the sample capacitor (Csamp). The first is a n-to-1 multiplexer that selects the channel to be converted. The second is the sample-and-hold gate that is controlled by the ADC’s successive approximation state machine. Rsamp is the internal charging resistance, and Csamp is the sample and hold capacitance.

    As soon as the ADC conversion starts, the sample-and-hold switch is closed, connecting the sample-and-hold capacitor to the analog input through the internal ADC resistance (Rsamp and Rmux). This causes a charging current to flow into the analog input and the capacitor starts to charge. The time the switch remains closed is decided by the fADC. It is called sampling time. 

    The maximum current (at beginning of charging) is determined by the source voltage and the impedance of the signal source plus ADC internal impedance (500ohms)(use Ohm's law). As the sampling proceeds, the capacitor's voltage gets close to the signal voltage thereby making the current much smaller. When sampling stops and the acquisition starts, this voltage difference should be less than one LSB.

  • What is the roll of on state bias current in this? In multiplexer only one channel is selected at a time.

    Does on state bias current of selected channel will also provide leakage to other channels? Or it just provide current to Cmux?

    Please help us to understand this.

  • Hi all,  for this query, doesn`t have reply for a long time, is any update?

  • Sorry for late response.  My understanding is that the on-state bias current is the leakage current After the Csamp is charged. Yes, it provides the leakage current for all other channels.