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Part Number: THS4130
Tool/software: TINA-TI or Spice Models
I have configured the THS4130 and THS4520 for single ended to differential amplifier, gain x1.
THS4520 has about 4.6nV and 5.8nV output noise.
THS4130 has about 27nV and 40nV output noise.
Both circuits (input, RF, RG) are mostly the same. See the attached TINA files.
Why is the THS4130 noisier? The THS41340 is expected to be lower noise.
Why is the positive and negative outputs different i.e. in terms of noise?
You have to measure the noise differentially, with a differential volt meter. The THS4131 has higher 1/f noise, which will dominate at lower noise analysis frequencies. Also, the THS4130 has more "peaking", or increased noise gain at the amplifier's max bandwidth response.
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In reply to Sean Cashin:
THS4520 at 10Hz is specified as about 5nV.
THS4130 at 10Hz is specified as about 7.5nV.
There is not a big difference at low frequency.
THS4130 has about 35nV output noise using a gain of x1 and low resistor values. Is the THS4130 model correct?
In reply to alpesh kavia1:
I finally noticed that VG1 has a 1kΩ source impedance; FDAs are similar to inverting amplifiers in that your source impedance affects the gain. Additionally, both sides need to have matched Rf and Rg to maintain the fully differential signal. In this case, you should add 1kΩ to the un-driven Rg and increase the value of both Rf by 1kΩ to maintain unity differential gain.
Next, amplifier outputs act like a synthetic inductance, based on the amplifier's open loop gain, and they start to create resonant frequencies when driving capacitors without sufficient isolation resistors. This will increase gain and noise contributions at those frequencies. I increased the output filter resistors from 10 to 1kΩ and it seems to decrease the resonance both amplifiers which will make a straightforward noise calculation more accurate.
Finally, the THS4520 has higher bandwidth, so more noise will be integrated unless a lower frequency filter is applied to both circuits.
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