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TINA/Spice/THS4521: Noise Bandwidth calculation and simulation to the THS4521 sigma delta driver

Idan Ozana
Intellectual 480 points
Part Number: THS4521
Other Parts Discussed in Thread: TINA-TI, , ADS1274

Tool/software: TINA-TI or Spice Models

Hi,

In my application I need to measured 24 bit analog input, low frequency signals. For that purpose, I thought to use the ADS1274 Simultaneous Sampling, 24-Bit Analog-to-Digital Converters with the THS4521 sigma delta ADC Drivers (according to the Figure 89 in the ADS1274 datasheet).

  1. Problem 1: In my application I'm using the THS4521 with gain 1 configuration which has 145MHz gain at 0dB. When I'm trying to calculate the total noise (broadband, flicker…), I'm getting very high noise values as "Noise Bandwidth" (BW sub n) so the total noise is very high (assume brick wall factor 1.57). I notice that in Figure 89 you used a 1.5nF capacitor for, I'm guessing, low pass filter, right? What is the cutoff frequency?
  2. When using the TINA TI simulation for noise analysis (attached herewith, figure 89), I need to connect the "rg1" left signal to something. I tried to connect it to Vcom (2.5V) but I getting errors in DC analysis first. How can I connect this leg in order to perform the Noise analysis?

Thanks,

Idan  sbom341d.tsc

  • 0
    TI__Expert 8670 points
    Hi,

    For your TINA simulation circuit, you should connect the non-signal input to ground in this case. For single ended to differential conversion with FDA's, you want to match the impedances between your signal and non-signal sides. In this case you have a source connected to ground with no internal resistance and no DC offset, which looks like a connection to ground from an impedance standpoint. Therefore you can just connect the other side to ground.

    For your noise analysis are you looking purely at the amplifier noise, or the total noise include the external resistors and capacitors. Also are you just using it in a gain of 1 V/V or as a filter like the TINA schematic.

    Regards,