New Bipolar Boost TX and Front End

[KingJL]

I do not think that I would be able to reliably handle the ADS8900B with my current setup, although spec wise it is more than adequate, and to tell the truth the VQFN packaging makes it less susceptible to noise/interference.

It would be nice if there were a small module available that could be socketed to our board that used the ADS8900B. In fact a small general purpose module would have many applications. I do like the enhanced SPI interface to reduce clocking speed needed to retrieve the data... but alas... I am not setup to handle it.

[KingJL]

It would seem that currently, the best option is the pin compatible LTC2378 /2380-20 devices which would give the design some flexibility for purchasing either part in future without compromising your design.
At some point the price will drop and as you have stated they are available here and there at good prices.

The TI device I have used in the past is ADS8900B only available in VQFN.

I do not think that I would be able to reliably handle the ADS8900B with my current setup, although spec wise it is more than adequate, and to tell the truth the VQFN packaging makes it less susceptible to noise/interference.

[Mdtoday]

Like you, In the past, I have generally used TI devices. One reason, until recently, I could always get samples for experimentation. But, for ADC's over 18 bits with sample rates >= 1 Meg, SAR seems the only way to go and the packaging usually precludes DIY. That was one of the reasons for selecting the LTC2378-20 (actually early on I was leaning toward the LTC2380-24)... 16 pin MSOP is manageable for DIY.

It would seem that currently, the best option is the pin compatible LTC2378 /2380-20 devices which would give the design some flexibility for purchasing either part in future without compromising your design.
At some point the price will drop and as you have stated they are available here and there at good prices.

The TI device I have used in the past is ADS8900B only available in VQFN.

[Mdtoday]

Yes, TI were great for sampling. Waiting 12 weeks and jumping through more hoops got tiring so I just go with Digikey, Mouser, Element14 etc, even though the prices are higher, it does not slow a project down to a crawl. I generally get given 6-8 months to produce a prototype so 12 weeks is out of the question commercially. My feeling is the US/China trade problem is part cause of the blow out in lead times and MOQ, hence up-shift in pricing from distributors.

I agree, MSOP is manageable for DIY

[KingJL]

... I generally use Ti devices but at 20 bits SAR the packages are probably not DIY friendly for most folk on this forum

Like you, In the past, I have generally used TI devices. One reason, until recently, I could always get samples for experimentation. But, for ADC's over 18 bits with sample rates >= 1 Meg, SAR seems the only way to go and the packaging usually precludes DIY. That was one of the reasons for selecting the LTC2378-20 (actually early on I was leaning toward the LTC2380-24)... 16 pin MSOP is manageable for DIY.

[Mdtoday]

A good source... NO. But occasionally I can find them in the more affordable range of $25-$35(US) as here. I don't know what is happening with pricing of this IC. Three months ago, when I was first considering it's use, I could find them at Mouser for < $50 US. Now Mouser is at $60 US (IMS version). Mouser should have the CMS version available late Feb... it is listed for $51US... status "on order". The ltc2380-24, which is pin compatible is available $5 cheaper. If we can find a cheaper alternative, I'm all for it.

I have had experience with TI's ADS8881 18 bit SAR. It might be acceptable if the preamp gain is >= 64, but I really think a minimum of 20 bits with less preamp gain to achieve maximum dynamic range and sensitivity.

Thanks JL. I think the LTC2378 is a great device and very well specified and agree, the 20 bit resolution and less preamp gain is the way to go. It's a shame the pricing is ramping up but I have noticed this across a lot of semiconductors this year and lead times moving right out to crazy times, I think that is probably the main driver on cost at the moment.
I generally use Ti devices but at 20 bits SAR the packages are probably not DIY friendly for most folk on this forum
Ill do some more research and see what I can find too

[KingJL]

A good source... NO. But occasionally I can find them in the more affordable range of $25-$35(US) as here...

and here at aliexpress.

[eclipse]

LTC2380-24 looks very nice. I've checked the catalogs and can't find good ones (and cheaper), except for the MAX11905ETP+ which is cheaper.
The difference in cost is not very high so LTC2380-24 is best price/specs (24Bit; 50Hz/60Hz Rejection; Integrated Digital Filter with Real-Time Averaging)

[KingJL]

Thanks JL, on another matter, I was looking up availability of the ADC LTC2378CMS-20-PBF and could not really find a supplier with stock and the price in AU$ is around $78 - $92.
Have you found a good source for this device? Mouser and Digikey have the IMS version but still around same pricing.

A good source... NO. But occasionally I can find them in the more affordable range of $25-$35(US) as here. I don't know what is happening with pricing of this IC. Three months ago, when I was first considering it's use, I could find them at Mouser for < $50 US. Now Mouser is at $60 US (IMS version). Mouser should have the CMS version available late Feb... it is listed for $51US... status "on order". The ltc2380-24, which is pin compatible is available $5 cheaper. If we can find a cheaper alternative, I'm all for it.

I have had experience with TI's ADS8881 18 bit SAR. It might be acceptable if the preamp gain is >= 64, but I really think a minimum of 20 bits with less preamp gain to achieve maximum dynamic range and sensitivity.

[Mdtoday]

You are correct, LT6203... I had already caught that and changed the library, but it did not update the value in the schematic... a Kicad quirk. Done!! Done... also added two more via connections for 5V from J1 pins 19 & 20.

In the schematic, the values for R4 & R5 are changed from 10k to 1k; and the value for R6 is changed from 5k to 500R.

I will post updated project files and gerbers after we have a chance for further checking to see if any other changes are needed.

Thanks JL, on another matter, I was looking up availability of the ADC LTC2378CMS-20-PBF and could not really find a supplier with stock and the price in AU$ is around $78 - $92.
Have you found a good source for this device? Mouser and Digikey have the IMS version but still around same pricing.

[KingJL]

just checking schematic against manufacturer data sheets and I couldn't locate the data sheet for an LT6302C device, is this meant to be LT6203C ?

You are correct, LT6203... I had already caught that and changed the library, but it did not update the value in the schematic... a Kicad quirk.

I ran a schematic DRC and came up with the following errors...which are not critical as connections on PCB are correct .
To remove the errors I did the following;

Add label RX to pin 1 of J1

Re-assign pins of U2 ...
Pin 8 = input
Pin 1 = input
Pin 11 = output

Re-assign pin 6 of U4 to input

Remove no connect symbol from J2 Pin 8

Done!!

@JL On the top side of PCB at J1 pin 12, the track width could be increased to .508 mm to match other GND pins/tracks and perhaps add some more vias in that area.

Done... also added two more via connections for 5V from J1 pins 19 & 20.

In the schematic, the values for R4 & R5 are changed from 10k to 1k; and the value for R6 is changed from 5k to 500R.

I will post updated project files and gerbers after we have a chance for further checking to see if any other changes are needed.

[Mdtoday]

@JL On the top side of PCB at J1 pin 12, the track width could be increased to .508 mm to match other GND pins/tracks and perhaps add some more vias in that area.

[Mdtoday]

@JL, I ran a schematic DRC and came up with the following errors...which are not critical as connections on PCB are correct .
To remove the errors I did the following;

Add label RX to pin 1 of J1

Re-assign pins of U2 ...
Pin 8 = input
Pin 1 = input
Pin 11 = output

Re-assign pin 6 of U4 to input

Remove no connect symbol from J2 Pin 8

I only mention this for clarity as my first round checking so far, has found no actual PCB to Schematic connection problems
Looks good! I will go over it again a couple of times to be sure.

[Mdtoday]

@JL, just checking schematic against manufacturer data sheets and I couldn't locate the data sheet for an LT6302C device, is this meant to be LT6203C ?

https://www.analog.com/media/en/technical-documentation/data-sheets/620234fd.pdf

[Mdtoday]

The new ADuM version of the TX/RX board required a new PCB to interface with the CMOD-A7 as the number of signal lines have changed. At the same time, I wanted to accommodate a future move to an LTC2378-20 ADC. Interfacing to a 20 bit ADC introduces some new challenges because of sensitivity to noise/interference. Interfacing with the LTC2378 introduces even more challenges because the LTC2378 and all support IC's are in MSOP packages. To reduce susceptibility to noise/interference and to insure short ground paths, a GND fill is applied to both the top and bottom boards and via stitching is applied with uniform spacing. Then the RX signal path to the ADC is "fenced" with gounding.
TX/RX IF board schematic:
[ATTACH=CONFIG]48834[/ATTACH]

The LTC2378-20 and all support circuitry is optional as the current signal going to the CMOD-A7 XADC is still present. The FPGA HDL code will test if the LTC2378 is present and change the internal signalling and timing.

With the ADuM version of the TX/RX design, the PMOD sockets are no longer needed and have been removed. There is still the PMOD on the CMOD-A7 that is available if a PMOD is needed.

Over the next few weeks, the new I/F board schematic, PCB layout, and gerber files need to be vetted.verified. In support of that ongoing effort, I am posting the schematic, the kicad development file zip, the gerber files zip for the interface board, and the IF board BOM.

Excellent work JL! Looking really good. I will spend some time going over the circuits and layout during the next few days and report back.
Thanks for sharing