4 Wideband Filters

Even though delta-sigma ADCs with sinc filters are a great fit for low-bandwidth applications, the delta-sigma architecture can also be used with great success in higher bandwidth applications as well. For example, precisely measuring audio or vibration signals can be done using delta-sigma ADCs. Test and measurement equipment that requires high bandwidth is another application well-suited for delta-sigma ADCs. For these applications, a wideband filter can be used as the integrated digital filter on a delta-sigma ADC.

AC applications require a wide passband with little attenuation or ripple, but still require the precision that comes standard with the oversampling topology of a delta-sigma ADC. That is why some ADCs are designed with a wide passband filter that has a steep transition band. The ADS127L01, a 24-bit delta-sigma ADC optimized for wide-bandwidth applications, has such a filter. Figure 4-1 shows the magnitude response of the filter up to the output data rate, f_DR, of the ADC, which is 512 kHz. Note how steep the transition band of the filter is around f_DR / 2 – the frequency where signals begin to alias into the passband. This serves to drastically limit unwanted signals or noise from folding over. The stopband magnitude never exceeds –116 dB until the response of the filter repeats around the modulator frequency, f_MOD, of the ADC.

Figure 4-1 Magnitude Response for the ADS127L01 Wideband Filter up to f_DR

For higher-bandwidth applications, the sinc filter is less adequate for a few reasons. First, the frequency response rolls off early and droops at relatively low frequencies. This limits the usable signal bandwidth. On the other hand, the wideband filter remains almost perfectly flat until around f_DR / 2, maximizing the usable signal bandwidth for any given data rate.

Second, aliasing is more of a concern for AC applications. The sinc filter is not ideal because it does not attenuate sufficiently at half the output data rate, f_DR / 2, of the ADC. Alternatively, the wideband filter features exceptional stopband attenuation, which minimizes aliasing.

For ADCs without integrated digital filters, options are limited for achieving such a steep anti-aliasing response. High-order analog anti-aliasing filters with steep roll-offs are notoriously difficult to design and are subject to component tolerances and temperature. Alternatively, it is possible to oversample the converter and digitally filter in the processor, but this comes at the cost of instruction cycles, which may be scarce in high-speed applications. A wideband filter in a delta-sigma ADC truly solves this problem with its built-in “oversample, low-pass filter, then decimate” topology.

There is a trade-off with all digital filters where superb frequency-domain performance is achieved in exchange for longer settling times. After applying a step function to the input, the digital filter of the ADS127L01 requires 84 output sample periods to settle to the final output. For this reason, wideband filters are not ideal for applications that cycle between multiple-signal source inputs.

Figure 4-2 shows the step response of the wideband filter of the ADS127L01.

Figure 4-2 Response of the ADS127L01 Wideband Filter to a Unit-Step Input

The filter has a linear phase response, which means that its group delay (the delay due to phase shifting) will be constant across frequencies. For this filter, the group delay is 42 output data samples.

The wideband digital filter in delta-sigma ADCs provides an ideal functional block for wide-bandwidth, precision-measurement signal chains where no multiplexing between different signal sources is required. The flat passband with low ripple ensures that no in-band signal content is lost, and the steep transition and excellent stopband attenuation provide flexibility in environments where anti-aliasing is critical. In addition, the precision expected from a delta-sigma ADC can still be enjoyed.