Input/Output Impedance Clipping Voltages

  • Thread starter Thread starter mcleung
  • Start date Start date
  • Tags Tags
    Impedance
Click For Summary

Discussion Overview

The discussion revolves around the issues encountered when using a DAQ card and a microcontroller triggered by the same signal from an Agilent 33210A signal generator. Participants explore the implications of impedance matching and voltage levels in this context, focusing on the technical challenges of signal integrity and triggering performance.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant reports that their microcontroller fails to trigger properly when connected to the DAQ card, suspecting that the DAQ's low impedance is clipping the high voltages.
  • Another participant confirms that connecting a 50 Ohm load to a 50 Ohm source reduces the open circuit voltage, suggesting that the signal voltage is halved when terminated into a matching impedance.
  • There is a discussion about the Vih specification for the microcontroller, with one participant noting the importance of ensuring the signal source voltage exceeds twice the Vih value.
  • Some participants express uncertainty about the ability to adjust the amplitude and offset of the Agilent function generator, which is fixed in its current setup.
  • One participant suggests using a high-speed comparator as a buffer to address the voltage level issues between the signal generator and the microcontroller input.
  • Another participant points out that the trigger output from the Agilent generator is not designed to drive a 50 Ohm load, indicating it is meant for logic inputs instead.
  • There are mentions of the specific input characteristics of the DAQ card, including its input impedance and protection limits, which may influence the design choices for signal interfacing.

Areas of Agreement / Disagreement

Participants generally agree on the technical principles of impedance matching and voltage levels affecting signal integrity. However, there are multiple competing views regarding the best approach to resolve the triggering issue, and the discussion remains unresolved with no consensus on a definitive solution.

Contextual Notes

Participants note limitations regarding the fixed nature of the signal generator's settings and the DAQ card's impedance, which may restrict potential solutions. The discussion also highlights the need for further exploration of the microcontroller's voltage specifications and the implications of using different output configurations from the signal generator.

mcleung
Messages
8
Reaction score
0
Hey guys,

I’m trying to do a simple experiment where both my DAQ (Data AcQuisition) card and my microcontroller trigger off the same signal.
The trigger source is from the Agilent 33210A which is a 50 Ohm impedance output.
From that source, I have connected it to both the AlazarTech DAQ card (50 Ohm Impedance) and my microcontroller (9S12E128) (Set to High impedance)
I’ve noticed that my microcontroller is not triggering properly. When I disconnect the DAQ card, it works fine.

I am suspecting that the low impedance on the DAQ card is clipping off the high voltages.
I’m wondering if you guys have any ideas on how to fix this. I wouldn’t want (But if needed then I will) to introduce opamps or other circuits because it will create delays in the trigger signal.
 
Engineering news on Phys.org
mcleung said:
Hey guys,

I’m trying to do a simple experiment where both my DAQ (Data AcQuisition) card and my microcontroller trigger off the same signal.
The trigger source is from the Agilent 33210A which is a 50 Ohm impedance output.
From that source, I have connected it to both the AlazarTech DAQ card (50 Ohm Impedance) and my microcontroller (9S12E128) (Set to High impedance)
I’ve noticed that my microcontroller is not triggering properly. When I disconnect the DAQ card, it works fine.

I am suspecting that the low impedance on the DAQ card is clipping off the high voltages.
I’m wondering if you guys have any ideas on how to fix this. I wouldn’t want (But if needed then I will) to introduce opamps or other circuits because it will create delays in the trigger signal.

Can you look at the waveforms with an oscilloscope?
 
berkeman said:
Can you look at the waveforms with an oscilloscope?

Yes I have. I sent my oscilloscope to high impedance, and I notice that when I plug the trigger line into my DAQ card the trigger signal gets reduced SIGNIFICATLY.

It drops down from about 4.11 V (peak to peak) to 2.77 V.
 
mcleung said:
Yes I have. I sent my oscilloscope to high impedance, and I notice that when I plug the trigger line into my DAQ card the trigger signal gets reduced SIGNIFICATLY.

It drops down from about 4.11 V (peak to peak) to 2.77 V.

Yes, that is how things work with a 50 Ohm source driving a 50 Ohm load. The open circuit voltage of the voltage source is reduced to half when it is terminated into a matching impedance.

You would need to find out what the Vih spec is for the logic you are driving, and set the signal source to > 2x that voltage.
 
berkeman said:
You would need to find out what the Vih spec is for the logic you are driving, and set the signal source to > 2x that voltage.

What is "vih"? I suppose you are referring to the logical HIGH and logical LOW values that the micro controller uses?

Also, I can't change the signal source, the Agilent function generator is fixed. (can't change the impedance, nor the voltage)
I can't change the impedance of the DAQ card either.
 
berkeman said:
Yes, that is how things work with a 50 Ohm source driving a 50 Ohm load. The open circuit voltage of the voltage source is reduced to half when it is terminated into a matching impedance.

You would need to find out what the Vih spec is for the logic you are driving, and set the signal source to > 2x that voltage.

Are you living in Newark Ca? I see from your HAM radio stuff below by neither link work. I live in Sunnyvale.

Back to the question, Depend of the speed of the trigger signal, if the rate is slow, maybe is safer to remove the 50ohm termination from the DAQ so the signal don't get loaded down. I just afraid by accident the DAQ is removed and the Microcontroller will be cooked by to over voltage.
 
mcleung said:
What is "vih"? I suppose you are referring to the logical HIGH and logical LOW values that the micro controller uses?

Also, I can't change the signal source, the Agilent function generator is fixed. (can't change the impedance, nor the voltage)
I can't change the impedance of the DAQ card either.

We wrote at the same time. Look at the Agil for another output to drive the controller separately. Then you can safely raise the voltage output.
 
yungman said:
Are you living in Newark Ca?

[hijack] Yep! PM me if you want more info. [/hijack]
 
I'm using a 500MHz aquisition rate on my DAQ.

berkeman, I found this on my microcontroler

V_CC 5.0
V_OH 4.2
V_IH 3.25
V_IL 1.75
V_OL 0.5
Gnd 0

**V_OH 4.2, not 5.2
 
Last edited:
  • #10
mcleung said:
What is "vih"? I suppose you are referring to the logical HIGH and logical LOW values that the micro controller uses?
Yes

Also, I can't change the signal source, the Agilent function generator is fixed. (can't change the impedance, nor the voltage)
I can't change the impedance of the DAQ card either.

Why can't you set the 33210A amplitude and offset? It is able to put out 10Vpp into 50 Ohms:

http://www.home.agilent.com/agilent...y=1000032746:epsg:dow&cname=AGILENT_EDITORIAL

.
 
  • #11
mcleung said:
I'm using a 500MHz aquisition rate on my DAQ.

berkeman, I found this on my microcontroler

V_CC 5.0
V_OH 5.2
V_IH 3.25
V_IL 1.75
V_OL 0.5
Gnd 0

Yeah, setting the 33210A to 2.77Vpp (with offset voltage half of that) will not meet those CMOS logic levels.

If you're stuck with the 33210A setting for some reason, you could use a high speed comparator as the buffer between the signal generator and your logic input...
 
  • #12
berkeman said:
Why can't you set the 33210A amplitude and offset? It is able to put out 10Vpp into 50 Ohms:

http://www.home.agilent.com/agilent...y=1000032746:epsg:dow&cname=AGILENT_EDITORIAL

You are looking at the output. Yes the output can go up to 10v p-p. I'm using the sync output as the output is used to output the function I want the Acquisition card to sync to.


yungman said:
We wrote at the same time. Look at the Agil for another output to drive the controller separately. Then you can safely raise the voltage output.
The Agilant fn Generator has:
BNC Output (Used)
BNC Sync (Trigger out-What I am having difficulties with)
BNC Trigger In (Also used)
BNC Modulation In - for other purposes.
 
  • #13
mcleung said:
You are looking at the output. Yes the output can go up to 10v p-p. I'm using the sync output as the output is used to output the function I want the Acquisition card to sync to.



The Agilant fn Generator has:
BNC Output (Used)
BNC Sync (Trigger out-What I am having difficulties with)
BNC Trigger In (Also used)
BNC Modulation In - for other purposes.

The Trigger Output is not meant to drive a 50 Ohm load. It is meant to drive logic inputs. The datasheet that I linked to lists it as having a "5V Zero-Going Pulse".
 
  • #14
berkeman said:
The Trigger Output is not meant to drive a 50 Ohm load. It is meant to drive logic inputs. The datasheet that I linked to lists it as having a "5V Zero-Going Pulse".

Trigger output is used to trigger other systems. I'm using it to trigger my DAQ card (Below) and the microcontroler.

TRIG IN (External Trigger) Input
Input impedance 50 Ω
Coupling DC only
Bandwidth (-3dB) DC - 250 MHz
Input range ±3 V
DC accuracy ±10% of full scale input
Input protection ±8V (DC + peak AC without external
attenuation)
http://www.alazartech.com/products/ATS9350_v_1_1b.pdf
 
Last edited by a moderator:
  • #15
mcleung said:
Trigger output is used to trigger other systems. I'm using it to trigger my DAQ card (Below) and the microcontroler.

TRIG IN (External Trigger) Input
Input impedance 50 Ω
Coupling DC only
Bandwidth (-3dB) DC - 250 MHz
Input range ±3 V
DC accuracy ±10% of full scale input
Input protection ±8V (DC + peak AC without external
attenuation)
http://www.alazartech.com/products/ATS9350_v_1_1b.pdf

If the trigger input is always 50 Ohms and not configurable, then you will need a beefy buffer to drive it. You could use a 74AC244-type gate to do it (or a single gate version).
 
Last edited by a moderator:
  • #16
berkeman said:
If the trigger input is always 50 Ohms and not configurable, then you will need a beefy buffer to drive it. You could use a 74AC244-type gate to do it (or a single gate version).

Ok thanks for your help!

This is what I feared is that I'd require a amp. Hopefully I can find one with response times in the 500+GHz range also or else I'll need to decrease my acquisition frequency.
 
  • #17
mcleung said:
Ok thanks for your help!

This is what I feared is that I'd require a amp. Hopefully I can find one with response times in the 500+GHz range also or else I'll need to decrease my acquisition frequency.

In the which-what frequency range?!
 

Similar threads

Impedance mismatch issue in receivers
  • · Replies 11 ·
Replies
11
Views
2K
Amplifier Circuit help, impedance matching
  • · Replies 20 ·
Replies
20
Views
4K
Calculating Input/Output Impedance w/ Parallel Resistors
  • · Replies 7 ·
Replies
7
Views
2K
Why is impedance given as a simple Ohms unit when it's frequency dependent?
  • · Replies 10 ·
Replies
10
Views
3K
LC Reactive elements cancel cause huge V drop. High voltage
  • · Replies 12 ·
Replies
12
Views
2K
Determine the output impedance of the circuit
  • · Replies 25 ·
Replies
25
Views
4K
Transmission line impedance question
  • · Replies 6 ·
Replies
6
Views
2K
Impedance matching of transmission line to source impedance
  • · Replies 15 ·
Replies
15
Views
5K
Thoughts for a low-power 60Hz 120Vrms signal generator please
  • · Replies 47 ·
2
Replies
47
Views
5K
RF cavity shunt impedance, PD and input power
  • · Replies 13 ·
Replies
13
Views
4K