PCB layout for THS4521 and ADS1278

Jonathan Edelson

Other Parts Discussed in Thread: ADS1278, THS4521EVM, THS4521, ADS127L01, TINA-TI

I am currently building a 1 off based on the ADS1278, following the ADS1278EVM. I would simply use the evaluation board however my application needs at least 6 channels buffered, and also needs the buffer gain adjusted to deal with a +-10V bipolar input.

I have already viewed the following:

https://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/108440

http://www.ti.com/lit/an/slyt336/slyt336.pdf

https://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/190521

http://www.ti.com/lit/ds/sbos458h/sbos458h.pdf

I have the following requests/questions:

1) Are gerbers available for the THS4521EVM, described in this datasheet: http://www.ti.com/lit/ug/sbou080/sbou080.pdf

2) The layout for the THS4521 amps as used in the ADS1278EVM seems much relaxed as compared to that on the THS4521EVM. Is this because of the low frequency use?

3) The buffer circuit used on the ADS1278 is also shown on the front page of the THS4521 datasheet. A simple DC analysis shows that a the buffer circuit will take a +-2.5V single ended input (Vin- held at 0, Vin+ is bipolar) and convert this to a proper differential output centered on the Vocm value; without exceeding the common mode limits of the amplifier. Is my analysis correct?

4) For low frequency use (below 100kHz) can I simply change the input (Rg) resistors from 1K to 4K to deal with my +-10V input range?

5) Can the 1.5nF capacitors be safely increased to roll off gain at a lower frequency? My highest frequency of interest is 25kHz, and I was thinking of using 15nF rather than 1.5nF.

Many thanks

Jonathan Edelson

over 9 years ago

0 Ryan Andrews over 9 years ago

TI__Guru 66446 points

Hello Jonathan,

I support the ADS1278, while the THS4521 is supported by our high-speed amplifiers team. I will do my best to answer your questions below. If you need more clarification, you may also consider posting directly on the High-Speed Amplifiers forum.

1. I would post this question directly on the High-Speed Amplifiers forum. If they are available, someone from their team should be able to help you.

2. Perhaps it might be, but I don't believe this was intentional for any reason. It never hurts to pay extra attention to good layout examples for best performance. I would recommend following the layout suggestions on the THS4521EVM to learn about specific device recommendations. Our new ADS127L01 also has an improved layout guidelines section with an example layout for both the amplifier and the ADC. You can use this as a reference when doing the layout for the ADS1278 input paths as well. Try to keep the input paths matched and close together for the most common-mode rejection.

3. This will depend on the supply voltage and Vocm for the THS4521. For a 3.3V unipolar supply, I believe you will see some clipping at the amplifier outputs. For a 5V supply, simulation looks ok as you described.

4. Yes, but more attenuation may be needed to avoid output clipping. Please confirm with TINA-TI simulation.

5. This should pose no consequence as long as the amplifier can drive this capacitive load. Check simulation to verify that the amplifier is stable and has adequate phase margin. From the ADC perspective, a larger cap helps to suppress the voltage transients produced by each sample and results in better distortion performance.

Hope this helps!

Best Regards,

0 Jonathan Edelson over 9 years ago in reply to Ryan Andrews

Prodigy 60 points

Thanks! Those answers help quite a bit.

I am looking at the ADS127L01 datasheet and will look at TINA-TI.

Jon

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PCB layout for THS4521 and ADS1278