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Part Number: ADS1278
I would appreciate any help with my problem.
I have a data acquisition system based on ADS1278 (clock 25MHz) and THS4521 buffers. Buffers are connected as in Figure 89 of ADC datasheet. Capacitor across ADC input is 100pF.
When I used 5.6nF (later tested with 1nF) capactor in bufer input, its power consumption was 40mA (!). After removing those capacitors current consumption decreased to 2mA, but I observed large oscillations on ADC input - approx 4Vp-p @ 6.3MHz. Could you advise me please if this is a normal situation and how should I approach the problem? Increase ADC input capacitor value?
Precision ΔΣ ADCs
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In reply to Ryan Andrews:
Hi Jaroslaw, Sorry for the delay in reply. Would it be possible for you to attach your schematic in-between the THS4521 and ADS1278? Do you have any series resistor at the THS4521 output while driving the 5.6nF capacitor? I don't think the large oscillations you are seeing are normal and could be due to the THS4521 oscillating. Something you can try to stabilize the THS4521 circuit is add an additional 1.5nF cap across the THS4521 inputs. Best Regards, Rohit
In reply to Rohit Bhat:
Thank you for reply. Please find below schematic of the buffer and ADC input as requested.
AIP(N) inputs are connected directly to the board input connector. Capacitors across ADC inputs are changed to 2.2nF and C42 and C46 are removed now. At the moment oscillation across ADC is much smaller but buffers consume around 20mA each. I will try later to increase the C15 to suggested value.
In reply to Jaroslaw Pulka28:
Have you tried to increase the input cap (C15) value from 100pF to 1.5nF or higher value, and seen whether the oscillation goes away? I think your circuit will oscillate as it is because of not enough phase margin in the loop gain unity gain frequency with 100pF cap.
For a 100pF input cap, your PM goes to about -1.7' meaning no phase margin leading to sustained oscillations in the circuit. The same PM improves to about 80' using a 5.6nF cap at the input, indicating a stable circuit. You can use the below circuit in TINA-TI to simulate stability of the THS4521 circuit, where the loop gain is Vout as shown below.
To understand more about stability of fully differential amplifier such as the THS4521, I would recommend you to refer the following video.
Please let me know if you have any questions.
Hello Rohit, Kai,
Thank you for your answers and suggestions. Regarding my circuit - this is what happens when one forgets about basics.
To describe my system a bit more - input of the THS4521 is connected to the output of the AD8476 on different board (low ohmic driver). Buffer output still oscillates with C8 (your schematic) set to 2.2nF. Removal of the capacitor solves the problem completely.
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