SN74AHC1G32 output swing is half input

mariano rotunno

Other Parts Discussed in Thread: ADS1675, SN74AHC1G32, SN74AHCT1G32

Hello,

i've built a pcb based on ADS1675REF board.

Actually board does not behave the way i was expecting, analyzing signals on PCB to see if something's wrong i've discovered that applying a cmos clock signal to ADS1675 ADC through SN74AHC1G32 results in a half peak-to-peal swing range signal at output of OR gate in respect to signal applied ad its input.

The schematic is at page 20 of 23 of the ADS1675REF User Guide. Just i want to know, Is that a wrong behavior? Maybe the OR port is faulty? What is the reason the OR gate is placed on path through pin 55 of ADC? it realizes a sort of buffer?

Thanks in advance.

over 12 years ago

0 Chris Cockrill over 12 years ago

TI__Mastermind 46480 points

The output of the OR gate should be 5V. What is the signal applied to the input?

0 mariano rotunno over 12 years ago in reply to Chris Cockrill

Intellectual 790 points

A CMOS standard clock signal 0 - 3.3V range. Vcc is 5V

0 Chris Cockrill over 12 years ago in reply to mariano rotunno

TI__Mastermind 46480 points

Hi Mariano

3.3V is not a good high for 5V CMOS inputs. Change to the SN74AHCT1G32 with TTL inputs. This may correct your problem.

0 mariano rotunno over 12 years ago in reply to Chris Cockrill

Intellectual 790 points

Hi Chris please let me understand 'cause the ads 1675ref user guide http://www.ti.com/lit/htm/sbau162 reports at page 17 in Bill of Material the usage of SN74AHC1G32DRLR

in the path through pin 55 of ADS1675 ADC.. so is that an error?

Thank you

0 Chris Cockrill over 12 years ago in reply to mariano rotunno

TI__Mastermind 46480 points

I could not tell what level the clock signal was from the schematic but if it is 3.3V then they need the SN74AHCT1G32.

0 mariano rotunno over 12 years ago in reply to Chris Cockrill

Intellectual 790 points

Hi Chris,

in the thread related to ads1675 board i've got this answer:

"The ADS1675REF EVM has the SN74AHC1G32 device on it (you can very the package has AGS top-side markings per datasheet). It is serving as a level shifter/buffer from the 3.3V level of the Opal Kelley controller board. The other device (AHCT132) mentioned should be OK; the only major difference is that it has a wider supply range - but that is not required for this application since we wish to buffer the clock to 5V levels (ADS1675 is connected to AVDD = 5V for the EVM)."

so for my point of view, if SN74AHC1G32 has an output that is HALF in amplitude of input signal i can assume this or gate is faulty..isn't it?

sorry but i'm not an expert in this field so i can have questions on obvious things.

Thanks

0 Chris Cockrill over 12 years ago in reply to mariano rotunno

TI__Mastermind 46480 points

Hi Mariano

If all your inputs are in spec and all inputs are connected and not floating then the output should be equal to Vcc. So yes the part must be damaged or have a poor connection.

The reason to switch to an AHCT is the the Vih for the AHC is 3.5V, so 3.3V input is not a good high for an AHC part with 5V VCC.

SO if you use the AHC part at 5V Vcc and the input is 3.3V then you are out of spec. However I dont believe that is the cause of the problem you are seeing. The part is probably faulty.

Logic

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SN74AHC1G32 output swing is half input