Rise time issue- contrast from the datasheet

[lerameur]

Hello,

I purchased a few chips to measure the rise time of a square wave.
What I am finding out is that the datasheet shows a very fast rise time, but I cannot event get close to their rise time.
The LTC6904 has a rise time of 3ns in the datasheet, I am measuring about 25ns
The Mic1555 has a rise time of 15ns in the datasheet, I am measuring about 80ns
I have tested about 10 different chip, hex comverters, Frequency generators Schnimtts trigger etc.. I always measure 10x slower then whatthe datasheet id showing.
I tried different probes on the scope (1 Gig). Very short leads.
Anyone has seen this issue ? DO you come close to the datasheet rise time value?

Ken

 

[barry]

HOW are you measuring rise time? Your problem is either in your test equipment or your layout. Or both.

 

[KlausST]

Hi,

Be sure the datasheet value is true.

For sure you know: As long as you don't write about your test setup ... nobody can validate your values.

Klaus

 

[lerameur]

Hello,

Here I am posting 2 pictures. My oscilloscope and the LTC6904, by default this chip output about 1khz
I did get 100ns rise time for a ne555 and thats what the datasheet mentionned. Thats the only chip I obtained the correct value.

Ken

 

[lerameur]

Hello,

I was only able to obtained the exacte rise time of 100ns from a ne555 chip.
I joined two picture of my circuit (LTC6904) and my oscilloscope.


Ken

 

[wwfeldman]

data sheets usually show how various parameters are measured
if you measure the rise time the same way the manufacturer did
AND you use the same measuring device
AND your measuring device is calibrated and the leads are arranged correctly and ... and ...

then you'll probably get the same result

you're using a 1 Gig scope - is that the bandwidth or the digitization rate?
to measure something that happens on the same order (3 ns)

you should find a 'scope that has a 10 Gig bandwidth and a 10 Gig digitization rate
then your instrument will be better than what you want to measure

- - - Updated - - -

the Tektronix TDS 1002 in your pictures has a 60 MHz bandwidth and a 1 Gig/s sample rate

a 60 MHz signal is reduced 3 dB on that scope
a 100 MHz signal is reduced more

what you are seeing is the response of the scope, not the response of the chip

 

[Audioguru]

The solderless breadboard has stray capacitance between the rows of contacts and wires all over the place. The wires also have series inductance. Capacitance and inductances slow things down.

 

[KlausST]

Hi,

The test setup is critical.

In your pictures I find a couple of things to improve:
* the power supply decoupling capacitor needs as short as possible at the DUT's supply pins. Maybe 5mm, directly on the DUT PCB, not via breadboard. In your picture I see about 100mm, this is too long. It surely will degrade risetime.
* Don't use the long GND wire of the probe. Use very short GND connection of both probe signals directly at the DUT. No wiring, not via bread board.

Klaus

 

[FvM]

We can expect about 6 ns intrinsic rise time for a 60 MHz oscilloscope. 25 ns in the shown measurement is still far of. Apart form the already mentioned problems of bread board setup, are you possibly using a 1:1 probe? It has huge input capacitance and considerably affects signal quality.

Tek P2200 probe is e.g. specified with only 7 MHz bandwidth in 1:1 setting.

 

[Easy peasy]

capacitance board forming LC low pass filters, you need very high quality probes to see rise times better than 2nS ...

 

[lerameur]

Hello all,

I adjusted my propde to 10x and now getting proper rise time. Thats for all your comments.
I also tried an older probe (pp-150) on channel #2 and I anm getting similar rise time but different waveform. see pictures attached.

Ken

 

[Audioguru]

It looks like the 10:1 scope probe is not correctly adjusted:

 

[KlausST]

HI,

your scope has an probe adjustment signal. Left side from the three BNC connectors.
Use a screwdriver at the probe to adjust for pure square wave.

Klaus

 

[FvM]

Apart from incorrect probe adjustment of channel 2 probe, both channels are showing some ringing due to too long probe ground cable or insufficient clock generator supply bypassing. It can be probably accepted if your primarily interested in the digital circuit function.

To see the real IC output signal, you need a faster oscilloscope and better probe. And better circuit design to get a good square wave.

 

[lerameur]

I adjusted my probe, working good now. The rise time are very close to the datasheet specs. Very happy. I am aware that the circuit can be improved.
For the probes, what probe do you recommend at a reasonable price?

 

[d123]

Hi,

In the words of - I believe I read it said by - Robert Kolmann (in another article): (sic) "Don't use the witches hat." Better measurements obtained with wire around ground ring on osc. probe tip to gnd point of interest and probe tip directly on voltage point of interest.

Example of this in figure 2:

Power Tip #6: Accurately Measuring Power Supply Ripple