AM3352: ADC input imperdance

Connecting a high impedance source to one of the ADC inputs may not be a good idea. Accuracy of your reading may be compromised with a high impedance source since it takes a long time to charge the ADC input capacitance during the short acquisition period. Also input leakage may also create an undesired voltage offset when using a high impedance source.

You may need to buffer the LT6003 output with an op-amp.

I will try to find and review the LT6003 datasheet tomorrow, then provide another reply with more information related to your question about the advisory.

Regards,

Paul

I reviewed the LT6003 datasheet and it appears to be a very low power op-amp designed for portable battery powered products.  I'm concerned the high output impedance of this op-amp may not be able to fully charge the input capacitance of the ADC during the acquisition period, which may cause measurement accuracy issues.  The acquisition period is programmable and can be increased to provide more time for the capacitor to charge, but this also can reduce accuracy of the measurement as frequency of the signal increases.

If the applied signal is very low frequency, they may be able to connect a de-coupling capacitor which will average the applied voltage and source the ADC input capacitance during acquisition.

Your customer needs to understand input characteristics of the ADC so they select the correct source impedance.  The ADC has about 5pf of constant input capacitance with an additional 5.5pF that is connected to the signal during the acquisition period.  They need to be sure the source impedance of any attached device is able to completely charge this capacitance in the acquisition period, which is defined by ADC clock frequency and value of the SampleDelay bit field in the respective STEPDELAY register. When SampleDelay is configured to its default value, the 5.5pF sample capacitor is connected to the selected input for two ADC clock cycles. This period can be increased one ADC clock cycle for each increment of SampleDelay.

The output impedance of the attached signal source also needs to be evaluated with respect to the unexpected power-up condition described in advisory 1.0.32.  This is mostly a concern if the unexpected power-up condition loads the output of the source such that the output current rating of the attached device is exceeded.  If this is the case, they may need to insert a series resistor to limit current during this condition.  However, the series resistance should be just large enough to protect the output of attached device during the unexpected power-up condition.  Otherwise, the series resistance may prevent the ADC input capacitance from being fully charged during the acquisition period.

Regards,
Paul

Hi Paul,

Thank you for your reply.

The output impedance of the op-amp depends on the frequency, so how many hertz and how many ohms or less should I choose for the output impedance of the op-amp connected to the AM335x?
I would like to know about the criteria for selecting an op-amp.

The customer's ADC setting values are as follows.
　ADC_ClkDiv = 15,
　SampleDelay = 0,
　CLK_M_OSC = 24Mhz,

Best Regards,
H.U