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# ADS1148: Input capacitance and resistance

Part Number: ADS1148

Hi,

Could you let me know input capacitance and resistance of each channel input of ADS1148?

Thanks.

• Hi David,

Are you looking for basic information like the pin capacitance and resistance of the ADS1148? If so, some of that detail is in the IBIS model located under the 'Tools & software' tab of the ADS1148 product folder. If you have a more complex need, based on some specific issue with sampling or input impedance that you or your customer is experiencing, please provide a little more detail.

Regards,

Tom

• In reply to Tom Hendrick:

Hello Tom,
Customer needs input impedance.
They are designing external filter and want to check how impact it to input impedance.
Customer already knows impedance will be enough high to be not impacted by external circuit.
But they just want to check it.

Thanks.
• In reply to David Park:

David,

Just to make sure that we've covered the answers, Tom mentioned that the input capacitance can be found through the IBIS model. For the ADS1148, the input capacitance is about 0.4pF and 0.7pF depending on the input channel.

For the input impedance, this is a bit more complicated of an explanation. In these ADCs, the input voltage is repeatedly sampled at a high clock frequency, that is the same as the modulator rate. The input current is often modeled as a periodic charge and discharge of the input sampling capacitors of the ADC. If the inputs are buffered, then there may be some charge and discharge of parasitic capacitances as the inputs often have an input buffer amplifier that is chopped. This may give the appearance of an input impedance, where the input current is proportional to the input voltage.

However, there may also be small amounts of leakage current due to the input ESD protections or from the input multiplexer. These currents may have some relation to the input voltage, but may not be proportional or linear to the input.

In general, we will give some sort of typical plots showing the input leakage current and other values in tables regarding the current. However, in the ADS1248 there aren't any plots. The typical differential current is listed as 100pA on page 9 in the Electrical Characteristics table. The datasheet also gives absolute input currents and effective input impedance for different data rates in Table 4 on page 21 of the datasheet. I would generally refer to this table for input impedance.

Hopefully this fully answers your question. I'll close out this post, but if you have further questions, you can add another post, or open up a new thread if you are locked out.

Joseph Wu