quick question about ADC input max voltage

antony_GB — Tue, 08 Feb 2011 18:01:32 GMT

Hello, can anybody tell me what the maximum input voltage to the MCF5213's ADC is. I assume it allows a degree of over voltage. Thank you. antony

Re: quick question about ADC input max voltage

scifi — Wed, 09 Feb 2011 04:23:48 GMT

You should have a look at the 'input injection current' spec of the ADC. The inputs of the ADC should be protected with on-chip diodes connected to power supply rails. Thus, the input voltage range extends one diode voltage drop above and below supply voltage. The input current should be limited to maximum allowed injection current. Use a series resistor if necessary, but consider the possible degradation of ADC performance.

Re: quick question about ADC input max voltage

TomE — Wed, 09 Feb 2011 14:25:14 GMT

> You should have a look at the 'input injection current' spec of the ADC.

Whch is in the MCF5213 Electrical Characteristics document, Table 34, Page 37:

IADI Input injection current per pin: 3mA

And read note 7 associated with that spec.

The Maximum (normal) voltages for the pins are Vrefl and Vrefh, but according to the Errata, they had better be connected directly to Vssa and Vdda or there are problems.

> The inputs of the ADC should be protected with on-chip diodes connected to power supply rails.

As indicated in the diagram in section 2.13.

> Thus, the input voltage range extends one diode voltage drop above and below supply voltage.

But the limiting specification is the injection current.

> The input current should be limited to maximum allowed injection current. Use a series resistor if necessary, but consider the possible degradation of ADC performance.

Both the frequency response and the voltage offset the resistor causes. "Figure 8. Equivalent Circuit for A/D Loading" in Section 2.13 should give you enough information to work this out.

There is also a HUGE amount of information on this subject in the MCF5282 User Manual (the one Freescale uses to describe the MCF5313). Read the sections

28.9.5 Accommodating Positive/Negative Stress Conditions

28.9.6 Analog Input Considerations

28.9.7 Analog Input Pins

28.9.7.1 Settling Time for the External Circuit

28.9.7.2 Error Resulting from Leakage

Tom

topic quick question about ADC input max voltage in ColdFire/68K Microcontrollers and Processors

quick question about ADC input max voltage

Re: quick question about ADC input max voltage

Re: quick question about ADC input max voltage