ADS122C04: Digital Input/Output Input Current

Hi Sakai-san,

See my responses below.

Best regards,

Bob B

hideyuki sakai said:

Part Number: ADS122C04

Dear, Sir.

My customer is considering to apply ADS122C04 for their system.

I would like to make sure several things as below, it is related to the digital

input/output input current. Please give your advice.

1. I think the pull-up resistor will be to limit the current at outout=Low(internal transistor ON).

   What is the meaning of "Input Current", -10uA/10uA? [Bob] This current is primarily leakage current through ESD input structures for input pins such as the RESET pin or address pins (A0 and A1).

   I wonder it is a leakage current at output=High(internal transistor OFF? [Bob] This would be leakage current, and is covering a large input voltage range and is not dependent on CMOS input structure which is very high input impedance.  Open drain circuits require a pull-up resistor which is sized to allow much larger currents to be applied (mA as opposed to uA).

2. If above was YES, higher pull-up resistor value would be caused for too much voltage drop

   on output HIGH level, so smaller value would be better. But the DVDD current consumption

   would become higher. I wonder my understanding is correct? [Bob] The pull-up resistor needs to be lower in value to provide fast switching times to overcome I2C line capacitance at higher I2C SCL frequencies.  Timing specifications for the ADS122C04  are given in the datasheet in section 6.6 on page 8 and are based on pull-up values of 1k Ohm.  A 1k pull-up resistor with 10uA leakage is only 10mV and will not affect the operation of the system.  Even a 10k pullup would not affect the operation of the system, although a resistor of this value would not allow for the maximum SCL frequencies to be used for maximum throughput.

3. The spec is defined under -40 to 125 deg.C.

   The custoner's system ambient temperature is just -20 to 50 deg.C.

   Is it possible to optimize the input current spec for finding the best pull-up resistor value to realize

   above #2? [Bob] As I stated earlier, the pull-up value is not related to the input current, but rather the SCL frequency of operation.  This information is in the datasheet at the top of page 48 just above the section 9.1.2.  All testing information in the datasheet is based on 1k pull-ups for SDA/SCL.  If pull-ups are used on address pins or the reset pin, the resistance values can be much higher.  In most cases the A0 and A1 pins can tied directly to DVDD or DGND and do not require pull-ups or pull-downs.  The RESET pin if not used can be tied directly to DVDD.  If the DRDY  pin is used, then this pin must also have a pull-up.  The value chosen will also depend on line capacitance of the trace.  Generally values of 10k to 47k can be used to limit power consumption.

Best Regards,

H. Sakai