LPC5502 A/D converter

[ianbenton]

The datasheet says in Table 38 that the A/D input resistance is 2k ohms.

The manual says in Table 727 that "Extending sample times allows higher impedance inputs to be accurately sampled".

If its input resistance is 2k, then it simply can't measure high impedance inputs, can it?

Please clarify.

[nxf77486]

Hi @ianbenton 

I apologize for not expressing myself in the correct way and cause a mis understanding.

Yes it is possible to use the high impedance like the 1M ohm and if you extend the STS this will allow to have an accurate sample for the high impedance output.

Regarding the log in issue, it might be something on the web site that could be causing this. But there is nothing wrong on your account.

[ianbenton]

Actually it says "resistance" not "impedance" (I misquoted it). Where is the other end of the 2k resistance connected? Presumably, about 1mA of DC current flows into the input - does it go to 0V, or to 3.3V or some other voltage reference? Is there an equivalent circuit of the input? I didn't see one in the manual.

[ianbenton]

But 2k is a low impedance. 1M is a high impedance. Are you saying that it is not possible to use high impedance inputs?

By the way, why do I get this error message whenever I log in?

An Unexpected Error has occurred.

•  
• Your request failed. Please contact your system administrator and provide the date and time you received the error and this Exception ID: 37FF74E2.

  Click your browser's Back button to continue.

Return to my original page

[frank_m]

> But 2k is a low impedance. 1M is a high impedance. Are you saying that it is not possible to use high impedance inputs?

The actual impedance of the ADC input is the named 2k internal impedance, plus the impedance of the source in series.

An SAR-ADC uses a sample&hold capacitor to probe the input voltage, which forms a RC element with the input impedance. The source (measured input voltage) must be able to charge/discharge this capacitor in the configured sampling time, or else you will get incorrect results.

[ianbenton]

Now that makes sense. Thank you,
2k must be the resistance of the multiplexer. So the total source resistance is 2k more than output resistance of the source. So the fastest acquisition time is correct for a total of 2k, and should be extended by a factor of (2k+Rs)/2k for higher values of Rs (source impedance).

[frank_m]

Yes, correctly.

In case of high-impedance inputs, you could use a buffer amplifier.

For my private projects I used to go cheap, and do level-shifting and buffering with one opamp.

[ianbenton]

I put opamps on every a/d input thinking that the a/d input looked like 2k to ground.

Now I can delete most of them!

Just an aside. . . I bought the 64-pin LPC5502 thinking it had 9 a/d inputs, and found that two of them can’t be used because they occupy the same pins as the SWD input. It pays to read the small print!

[frank_m]

I mostly use stripboard for those projects, and larger schematics quickly become a pain in the back  hand-soldering everything. I'm more of the SW guy anyway.

> Just an aside. . . I bought the 64-pin LPC5502 thinking it had 9 a/d inputs, and found that two of them can’t be used because they occupy the same pins as the SWD input. It pays to read the small print!

It definitely does. But you could use the SWD pins. There are quite a few threads here (and on other MCU fora), exactly about that topic.

At least for the projects I had been involved commercially, going so cheap on debug capabilities never payed off. But for the management and accountancy guys, the two cents saved usually offset the developer's pain.

[nxf77486]

Hello @ianbenton ,

The Table 727 symbol STS is for the Sample time select bit 10:8, which explain that a shorts sample time will maximize the conversion speed for the lower impedance inputs. And if you extend the STS this will allow to have a an accurate sample for the high impedance inputs. So the recommendation is if you are using a 2k ohm  as an input impedance as is the max value show on the table 38, is to extend the STS for a more accurate sampled.