Buffer circuit to place between AD744 ouput and ADS1675REF THS4503 input front end circuit

the actual input front end of ADS1675REF board seems a low pass filter stage... Can it be capable to read 2us wide semigaussian pulses?

What about 80ohm resistore between two inputs and 750pF capacitor between two outputs? can you please suggest me how to modify this?

Thanks in advance

Hi Mariano,

I've attached a TINA circuit that I think will do the trick based on your description.

2625.THS4503 HPF + Attenuation and Level Shift.TSC

This circuit will attenuate your signal from 15V to 5V  for the input of the ADC, as well as set the appropriate output common mode level for the ADC.  It also maintains the ~16kHz high pass response you described from the 10nF + 1kohm combination.  I suspect based on your description that your DC levels were not set correctly, which is why you could not see your signal at the output of the THS4503.

For future reference, there is a very useful series of Application Notes on attenuating and level shifting signals using fully differential amplifiers:

Part 1: Differential Bipolar Input Signals

Part 2: Single Ended Bipolar Input Signals

Part 3: Single Ended Unipolar Input Signals

Let me know if you have any further questions -- 

Hi Bart,

thanks a lot for your help!

The circuit you've attached is very useful for my task and does exactly what i need, but  i've noticed that is different in component's placement about Test point, based on what probe's manufacturer told me.

In output of AD744 manufacturer told me that there is first a 10nF capacitor, than a 1k R though GND, and from that C-R node a 4.7K resistor that represents the test point.

If i'm right I've understood the way you shift sine wave around 7.5V to simulate pulses in 0, +15V interval on AD744 output..

But now i wonder how it can be possible to have pulses in the range (0,+15V) having this last (C-R)-R net where there is the 10nF capacitor that does AC coupling so that 7.5VDC offset to shift sine wave in input of AD744 is not available on test point. May it be possible to still have this kind of pulses in this configuration or the manufacturer gave me bad circuit description?

In my previous post when i've said that i cannot see the pulses i was referring to placing the oscilloscope's probe on test point when applying test point directly on input of THS4503 default ADS1675REF circuit.

Thanks again :)

I see.  It wasn't clear to me whether you had the flexibility to change these values.  Obviously I assumed that you could.  Here is an alternative that I think will work around your fixed values.  If my representation of your probe circuit is incorrect, let me know, and perhaps you could attach a schematic of the correct interface circuit.

6765.THS4503 HPF + Attenuation and Level Shift (2).TSC 

Hi Bart,

i really appreciate your help!

I cannot give you the schematic of the probe simply 'cause the manufacturer give me only the "description" of the output stage involving AD744, the same i gave you above, and so i have to solve issues basing only on this description. Maybe i have to open probe's hardware to see directly with my eyes what's in there.

The circuit you gave me perform exactly what i need, but still what i don't understand is:

if the probe's output has that 10nF capacitor that clearly performs an AC coupling, how it's possible that (not considering THS4503 at the moment), if i place oscilloscope on that TEST POINT i see pulses between 0 and 15V rising and falling on a 0VDC background? there must be something different on that output stage or i'm missing something important from my point of view...

Hello,

just want to explain in a few words what i think: That Vin' waveform must be visible just after the 4,7k Rg probe resistor's..

I can't say why you wouldn't be able to see anything, based on the description of the probe output you have given.  If you are probing at the test point with a high impedance probe (~1Mohm), there should be essentially no attenuation caused be the 4.7kohm resistor.  I think the THS4503 circuit I provided should work, but it sounds as though you have an issue before even getting to that point that you will have to address.

Sorry, I obviously misread your earlier statement.  You are seeing a 15V signal as expected, but not AC coupled.  Though again, based on your description, you are correct, the signal should be AC coupled at that point, so it sounds like you need a more accurate description of the output of your probe before you can proceed to the ADC interface.

Hi Bart,

finally i've received the circuit i was looking for! :) so i can now attach the schematic of that.

Hello,

i'm trying to simulate that circuit with THS4503 and i get convergence error... but if i detach AD744 circuit from ths4503 and place a probe on the output of AD744 i can see pulses like in the real world

Mariano,

It doesn't look like this circuit will work with the protection diodes as configured.  After the signal is AC coupled, you have a +/-7.5V signal, but the diodes clamp from 0V to 15V, so you will clip the bottom half of your signal.  I have shown 2 cases in the attached TINA simulation.

3107.THS4503 HPF + Attenuation and Level Shift (2) with Diodes.TSC

1) The lower diode is connected to ground, as your schematic shows.  Here you can clearly see that the signal is properly AC coupled at Vout2, but it is clipped at ground at Vout3.

2) I have connected the lower diode to -15V instead of ground.  The signal now has sufficient head room after being AC coupled, and the circuit works properly.

Connecting this diode to -15V instead of ground is one way to make this work.  Another easier option would be to remove the diodes altogether.  Since you're interfacing directly to the Test Point node, protection diodes are probably extraneous anyhow.

Hi Bart,

excuse me if i don't get you but using probe's schematic also with protection diodes placed between +15V and GND i can reproduce in some way the real pulses i can see with placing oscilloscope's probe on that test point. 

i'm trying to do another thing now, i want to simulate the test point circuit simply placing a voltage source with an internal resistance of 14,7k (i suppose is this the equivalent thevenin resistance of test point).

I want to use a  lmh6321 buffer to interface the THS4503.

Following the rules written in part3 single ended input signals you gave me in the beginning and looking at page 20 example formulas I've used a Rs=Rf=1K and have calculated Rt+=666ohm and Rg+=600ohm... Rt- seems to be 0Ohm so for Negate input i have only Rg-=1k.

Placing a square wave source with a 7,5V DC level and with 7,5V Amplitude before Rs+ i can simulate pulses with 0,+15V amplitudes and i've finally obtained a 

Vdiff square wave of 6Vpp (+-3Vref as needed from ads1675).

But what i can't really understand is that when i place lmh6321 just before Rs+ as input Vdiff simply drop to zero!.. and at that point also if i remove LMH6321 and place again the voltage generator as before adding in TINA lmh6321 i also get 0V on Vdiff! 

This Seems really strange! is it perhaps a TINA Software Bug?

I attach 2 file: circuit without LMH6321 with a voltage source with internal resistance of 5ohm as in lmh6321 specs that works fine, and circuit with lmh6321 that results in no output in THS4503 Vdiff..

Thanks .

I really appreciate your help!

MR

sorry this is 1st

this is 2nd