how to design an anti-aliasing filter in the front of ADS1118

Hi,


When putting together the anti-aliasing filter for this device you don't need to go as low as the data rate. Look at the frequency response of the ADS1118 in the datasheet with Figure 25 (which is for the lowest data rate). Based on the ADC you'd only need a cutoff frequency of about 125kHz, because the device input sampling rate is much higher.

As for a spice model, I'm not aware of any PSPICE model. However, an IBIS model is available:

www.ti.com/.../sbam083


Joseph Wu

When using delta-sigma converters the requirements for front end anti-aliasing filters are reduced to half the modulator sample rate, which for this device seems to be 250kHz (see Analog inputs section on page 15), therefore the cutoff at half of that frequency ends up being the 125 kHz number mentioned above.

As always though if you want higher performance, there is nothing wrong with adding extra filtering or lowering the cutoff frequency to any number you desire, it simply adds cost and complexity to your circuit and needs to be weighed out in the design process...the benefits of the added filtering may be worth it if your signal has a high degree of noise pollution from other sources (lower in frequency) that need to be filtered out. Page 29 seems to provide a substantial front end filter that you could probably use right away between a decent op amp and the converter...again added cost, but the choice is up to you and your requirements.

There is a lot of information on how sigma-delta converters work which explains the theory...all worth reading to understand the front end requirements and how they differ from SAR converters. Perhaps start here: www.ti.com/.../slyt423.pdf

Jason

Hi,


First, thanks Jason for adding your comments.

The ADS1118 is implemented basically as a delta sigma (or oversampling) data converter. Regardless of the data rate, the input is actually sampled at 250kHz to make each output data word. The input sampling is described in Figure 25 of the datasheet (but in the context of input impedance). Because it is repeatedly sampling the input over the entire data period, it will almost appear as "averaging" the input sample over that period of time.

I referenced figure 25 with the 8SPS data rate because other data rates will be similar. Since the data rate is 8SPS, the first zero in the frequency response will be 8Hz, higher frequencies will continue to attenuate all the way out to 1/2 of the modulator sampling frequency of 250kHz. If you used a higher data rate, you'd get the first zero at the new data rate. However you'd continue to attenuate out to the same frequency of 1/2 of the modulator sampling rate.

In comparison, a SAR ADC is more of a sample and hold type of circuit and the input is sampled once per data period, making the anti-aliasing filter important to 1/2 of the data rate frequency.

Going back to the ADS1118, there's nothing that requires you to use the 1/2 the modulator sampling rate. However, there might be other factors to consider. If you want to remove 50/60Hz noise, or if your input has a higher amount of lower frequency noise as Jason mentioned, you might need a lower frequency filter. I'm just saying that you don't need to go as low as half of the data rate for anti aliasing.



Joseph Wu