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Technical Discussions
Klein
Klein
03:49 Mar-18-2005
Ultrasound - Problems with AD600 ground reference

I’m a Brazilian researcher designing an ultrasound range finder for Biomedical Department. This is a picture from Orcad project schematic:
http://vesatur.sites.uol.com.br/temp/schematic.jpg
I placed an AC generator (Vin) to simulate the ultrasound reflected wave with 1mV amplitude and 700kHz.
This is the simulation: http://vesatur.sites.uol.com.br/temp/simulation.jpg
Gain = 41dB.

I’m designing an ultrasound receiver with AD600 (Analog Devices) to amplify the reflected wave with a preamp supplied by an OPA350. I’m really confused with all those AD600 ground connections and I’m aware if I make a mistake it will produce a lot of noise and it will interfere in my measures. The AD600 IC provides separated ground references for input (A1LO/A2LO) and output (A1CM/A2CM). Datasheet warns: “It is important that A1LO and A2LO be connected directly to the input ground. Significant impedance in these connections reduces the gain accuracy. A1CM and A2CM should be connected to the load ground.”
I’m using the same single transducer element for transmission and reception then transducer has to be connected with transmitter and receiver circuits. In my schematic, I labeled ground plane with number 1. These are my ground considerations:
• Label 1 - I attached all transmitter components and DC regulators connected to VPOS (pin 13) and VNEG (pin 12) in AD600 to ground plane.
• Label 2 - I presumed that I should connect the input grounds (A1LO – pin 3 /A2LO – pin 6) and gate input (GAT1 – pin 4/ GAT2 – pin 5). At the same reference, I connected the voltage limiter made by 1n4937 diodes and the preamp ground connection.
• Label 3 - I connected the output ground pins (A1CM – pin 15 / A2CM – pin 10) with the load resistor as an output ground reference.

Where should I connect the transducer? There are 2 options: the ground plane with the transmitter components or with the AD600 input ground reference.
Should I isolate AD600 ground reference from next stage (buffer + peak detector)? Is it safe connecting peak detector and AD ground output to ground plane?

I’m so sorry for my poor English.
Thanks in advance,

Claudio Klein



    
 
 Reply 
 
Linas Svilainis
R & D,
Kaunas University of Technology, Lithuania, Joined Nov 1998, 67

Linas Svilainis

R & D,
Kaunas University of Technology,
Lithuania,
Joined Nov 1998
67
07:30 Mar-18-2005
Re: Ultrasound - Problems with AD600 ground reference
Hello Claudio,

do not worry about the grounding in the way you do. Here are some general rules:
1.Every stage has filtering capatitors, or, as in AD case even output ground. These should be connected with as short as possible(and as thick as possible) copper traces to next stage input ground
2.All the grounds you have mentioned should connect to a single ground lane or at leas a copper pour, riveted in places where it is cut by other traces.
3.Power supply should connect to ground plane, closer to the output stage.

What you should worry is the connection of transducer. If possible, place the preamp as close as possible to transducer, or, even better, use differential xmission line (say, FTP cable). If connecting using asymetric (coax) line use same rule as above.
The other thing to worry is the V and I noise for preamp and optimal Rin, calculated from noise voltages.
For your case it's about 1MEG, are you sure you need this?

regards,

Linas


----------- Start Original Message -----------
: I’m a Brazilian researcher designing an ultrasound range finder for Biomedical Department. This is a picture from Orcad project schematic:
: http://vesatur.sites.uol.com.br/temp/schematic.jpg
: I placed an AC generator (Vin) to simulate the ultrasound reflected wave with 1mV amplitude and 700kHz.
: This is the simulation: http://vesatur.sites.uol.com.br/temp/simulation.jpg
: Gain = 41dB.
: I’m designing an ultrasound receiver with AD600 (Analog Devices) to amplify the reflected wave with a preamp supplied by an OPA350. I’m really confused with all those AD600 ground connections and I’m aware if I make a mistake it will produce a lot of noise and it will interfere in my measures. The AD600 IC provides separated ground references for input (A1LO/A2LO) and output (A1CM/A2CM). Datasheet warns: “It is important that A1LO and A2LO be connected directly to the input ground. Significant impedance in these connections reduces the gain accuracy. A1CM and A2CM should be connected to the load ground.”
: I’m using the same single transducer element for transmission and reception then transducer has to be connected with transmitter and receiver circuits. In my schematic, I labeled ground plane with number 1. These are my ground considerations:
: • Label 1 - I attached all transmitter components and DC regulators connected to VPOS (pin 13) and VNEG (pin 12) in AD600 to ground plane.
: • Label 2 - I presumed that I should connect the input grounds (A1LO – pin 3 /A2LO – pin 6) and gate input (GAT1 – pin 4/ GAT2 – pin 5). At the same reference, I connected the voltage limiter made by 1n4937 diodes and the preamp ground connection.
: • Label 3 - I connected the output ground pins (A1CM – pin 15 / A2CM – pin 10) with the load resistor as an output ground reference.
: Where should I connect the transducer? There are 2 options: the ground plane with the transmitter components or with the AD600 input ground reference.
: Should I isolate AD600 ground reference from next stage (buffer + peak detector)? Is it safe connecting peak detector and AD ground output to ground plane?
: I’m so sorry for my poor English.
: Thanks in advance,
: Claudio Klein
------------ End Original Message ------------




    
 
 Reply 
 
Cláudio Klein
Cláudio Klein
09:00 Mar-18-2005
Re: Ultrasound - Problems with AD600 ground reference
Hi Linas, first of all thanks for your help!

: do not worry about the grounding in the way you do. Here are some general rules:
: 1.Every stage has filtering capatitors, or, as in AD case even output ground. These should be connected with as short as possible(and as thick as possible) copper traces to next stage input ground

That's ok. I place capacitors to cancel DC offset voltages in all input stages.

: 2.All the grounds you have mentioned should connect to a single ground lane or at leas a copper pour, riveted in places where it is cut by other traces.

If it's safe (less noise) to attach all ground together at the same ground lane, it's the best for me!

: 3.Power supply should connect to ground plane, closer to the output stage.

I didn't understand this one. What's the reason? If I'm connecting to ground plane there is difference if I connect closer?

: What you should worry is the connection of transducer. If possible, place the preamp as close as possible to transducer,or, even better, use differential xmission line (say, FTP cable). If connecting using asymetric (coax) line use same rule as above.

I'm afraid I'm not doing this type of connection. I used a 50ohms coaxial cable soldered in a female BNC (where I connect the transducer) and this cable soldered directly on the copper board. Is it a big mistake?

: The other thing to worry is the V and I noise for preamp and optimal Rin, calculated from noise voltages.
: For your case it's about 1MEG, are you sure you need this?

I'm really not sure at all. I planned to project a preamp stage as an active filter with cut-off frequency at 876kHz (my transducer ressonat frequency is 700kHz). Is it generating a 1Meg Rin? I think it's better consider a simple unity gain preamp, isn't it?

Best regards,

Cláudio Klein

----------- Start Original Message -----------
: Hello Claudio,
: do not worry about the grounding in the way you do. Here are some general rules:
: 1.Every stage has filtering capatitors, or, as in AD case even output ground. These should be connected with as short as possible(and as thick as possible) copper traces to next stage input ground
: 2.All the grounds you have mentioned should connect to a single ground lane or at leas a copper pour, riveted in places where it is cut by other traces.
: 3.Power supply should connect to ground plane, closer to the output stage.
: What you should worry is the connection of transducer. If possible, place the preamp as close as possible to transducer, or, even better, use differential xmission line (say, FTP cable). If connecting using asymetric (coax) line use same rule as above.
: The other thing to worry is the V and I noise for preamp and optimal Rin, calculated from noise voltages.
: For your case it's about 1MEG, are you sure you need this?
: regards,
: Linas
:




    
 
 Reply 
 
Linas Svilainis
R & D,
Kaunas University of Technology, Lithuania, Joined Nov 1998, 67

Linas Svilainis

R & D,
Kaunas University of Technology,
Lithuania,
Joined Nov 1998
67
07:35 Mar-21-2005
Re: Ultrasound - Problems with AD600 ground reference
: 1.Every stage has filtering capatitors, ...
: That's ok. I place capacitors to cancel DC offset voltages in all input stages.
Sorry for misleading you. I ment power filtering capacitors.
Every active component is using the power decoupling capacitors to supply the current for high speed signals. So, you should follow the output stage path along to make sure every output has a capacitor supplying the current for next stage.

: : 2.All the grounds you have mentioned should connect to a single ground lane...
: If it's safe (less noise) to attach all ground together at the same ground lane, it's the best for me!
Not that straight. All the components placement should be done so, that returning current from one stage is not interfering with returning current for other stage.


: : 3.Power supply should connect to ground plane, closer to the output stage.
: I didn't understand this one. What's the reason? If I'm connecting to ground plane there is difference if I connect closer?
Just because the current consumption is highest in output stages, so it should not flow through input stages loops.

: : What you should worry is the connection of transducer. If possible, place the preamp as close as possible to transducer, or, even better, use differential xmission line (say, FTP cable). If connecting using asymetric (coax) line use same rule as above.
I ment point No1.

: I'm afraid I'm not doing this type of connection. I used a 50ohms coaxial cable soldered in a female BNC (where I connect the transducer) and this cable soldered directly on the copper board. Is it a big mistake?
Not that big - it depends on how much of pickup noise you are expecting. The low frequency signals shielding by conventional coax cables is very ineffective. Symmetric line has much better resistance to pickup noise.
In your design intrinsic noise is also high.
Let's do few simple calculations:
Together with two stages of AD600 you will have 80dB gain and about 25MHz bandwidth. Even taking into account AD600 V noise density, you will have 70mVRMS or 500mV p-p at the output. What's the point placing low-pass filter at the input, if noise generated at the output will be spread over 25MHz bandwidth? You should place the filter after last gain stage. Preferably - the passive one, since the active filter will introduce distortions (note the p-p value). But OPA350 has 4 times larger V noise, so noise at the output when max gain will be 2Vpp! Compare it with your ADC input range...
Even placing a decent low-pass (better-band-pass) filter at the output you still have 20mV pp noise.
Are you sure you will need all 80dB gain? if not, limit your choice to one AD600 or even replace it with AD604(with internal very low noise preamp) or AD603 (with programmable bandwidth)- these will give you 60dB ain.
Only after implementing the forementioned modifications you can start thinking of grounding and shielding.

: : The other thing to worry is the V and I noise for preamp and optimal Rin, calculated from noise voltages.
: : For your case it's about 1MEG, are you sure you need this?
: I'm really not sure at all. I planned to project a preamp stage as an active filter with cut-off frequency at 876kHz (my transducer ressonat frequency is 700kHz). Is it generating a 1Meg Rin? I think it's better consider a simple unity gain preamp, isn't it?
No, preamp gain should be above 6dB, othervise it's almost useless. But again - preamp is usually placed as close as possible to the sensor. In your case it seems you have it in main block. 1MEG is the OPTIMAL input rezistance, i.e. the source resistance value at which noise generated by input source resistance is dominating over silicon noise. Also, I do not see the justification why you have used rail-to-rail for preamp - these usually exhibit poorer CMRR...
Also, I am curious why C5 is 0.01uF but C11 is 0.1uF and C10 1000pF? It seems you have mentioned your center frequency is 700kHz ...




    
 
 Reply 
 

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