This thread has been locked.
If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.
Part Number: THS4521
I'm using the THS4521 for active 2-stage Bandpass Filter (4.14 KHz < f < 40.89 KHz), generated using FilterPRO.
This is the last part of analog receiver chain, that includes 4 stages: Instrumentation Amplifier (Gain of 300 [V/V], AD8429/Analog Devices) --> Single-Ended Input to Differential Output Amplifier (Gain of 2 [V/V], THS4521/TI) --> Active BPF (Gain of 1 [V/V], See above picture) --> A/D Converter (ads1274/TI).
In order to study the source of the noisy input to the ADC, the inputs of the Instrumentation Amplifier (INA) were shortened to ground.
The measured output signal of the INA is 30 [mVpp] (see F2 in picture #1), and it rises to 100 [mVpp;18 MHz] at the output of the Differential Output Amplifier (see F1 in picture #1). This amplifier is designed with 1 [kohm] series resistor and 2 [kohm] feedback resistor.
The measured signal at the middle stage outputs of the BPF is 714 [mVpp] at frequency of 60.98 [MHz] (see F2 in picture #2), and the signal at the last output of the BPF is 120 [mVpp] at frequency of 16.34 [MHz] (see F1 in picture #2).
In short - the BPF has large output noises at frequencies that do not appear anywhere at the circuit, and should be attenuated.
I suspect that the noisy output of the BPF is due to Bias currents of the THS4521, which are generated by the presence of the AC-Coupling on the inputs of the Op-Amps. Does it make sense?
Should the Op-Amps inputs have reference to ground, using large pull–down resistors?
Moreover, since I didn’t see any application using resistors with large values, I changed the feedback resistors of the first stage to 10 [kohm]. No improvement was observed in the results.
If the BPF picture is not clear, I can send it to your email.
I did have problems reading the schematic. You can email me a copy at loren dot siebert at ti dot com. The capacitor C159 should have some resistance between it and the amplifier output. I see that there are place holders for resistors, you could try using 25 Ohm capacitors as a start. I can simulate your circuit once I have a better copy of the schematic.
We are glad that we were able to resolve this issue, and will now proceed to close this thread.
If you have further questions related to this thread, you may click "Ask a related question" below. The newly created question will be automatically linked to this question.
In reply to Loren Siebert 1:
A copy of the schematic is sent to you by email.
I hope to hear from you soon.
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with respect to these materials. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.