FEATURE DATA ACQUISITION
architecture, representing a carefully chosen balance between stopband rejection, and settling time. The LTC2512-24 digital filter has a “no compromises” 0.001dB passband flatness extending to fo/4 (half a Nyquist zone). The LTC2512-24 filter transition zone and stopband attenuation are less aggressive than many delta sigma filters, allowing for faster settling and smaller time- domain artifacts. Once again, it is the absence of tones that makes such a filter practical.
The LTC2508-32 offers four pin- selectable decimation filters, with their properties listed in Table 1. The four different decimation filters allow designers to trade off between noise and bandwidth depending on the choice of application. For each configuration of the LTC2508-32, the digital filter is a low-pass finite impulse response (FIR) filter with linear phase response. The output of the digital filter is then down-sampled by the corresponding down-sampling factor (DF). Therefore, the resulting output data rate (fO) is equal to fSMPL/DF. In each of the decimation filter choices, the -3dB bandwidth is inversely proportional to the selected DF value. Each
Table 2. Properties of filters in LTC2512-24
configuration provides a minimum of 80dB attenuation for frequencies in the range of fO /2 and fSMPL – fO /2. For every 4x increase in the down-sampling factor, the ADC dynamic range increases by approximately 6dB, resulting in a dynamic range from 131dB at DF=256, up to 145dB at DF=16384. This is equivalent to an effective number of bits (ENOB) of 24 bits. Note that it is important that the ADC result is limited by a quality noise process, i.e., thermal noise, not its quantisation noise. This means that the ADC should provide at least a few bits more than the ENOB, and with a 32-bit output word, the LTC2508-32 provides a sufficient number of bits to represent the filtered data using an integer number of bytes. The output of the LTC2508-32 is
Figure 2: Magnitude of frequency response of the LTC2512-24 Digital Filter. fO = fSMPL/ DF
always fully settled in ten output samples, regardless of DF. As down- sampling factor is increased, bandwidth is decreased, limiting the noise and subsequently increasing dynamic range. With a down-sampling factor of 256, the -3dB bandwidth of the filtered output is 480Hz, resulting in an output data rate of 3906sps. At the highest DF of 16,384, the -3dB bandwidth is a narrow 7.5Hz, providing the highest filtering of the noise, and resulting in
the slowest output data rate of 61sps. In addition, the LTC2508 offers dual data output streams, the 32-bit digitally filtered version of the input signal, and a no-latency, 22-bit composite output directly from the front-end SAR converter. The no- latency output word consists of a 14-bit code representing the differential input, and an 8-bit code representing the input common mode voltage. The no- latency output is particularly useful in control applications, for fast tracking of the input signal and providing immediate feedback for changing load conditions. It can also be used to monitor the quality of the incoming signal and indicate system faults, for example, by detecting excessively noisy or oscillating signals that may be hidden by the digital filter. This information can be used in combination with the 8-bit common mode value for predictive maintenance. Changes in common mode voltage may point to trouble upstream, leading to potential equipment failure. To the designer, this looks like two ADCs in one, offering perfectly matched representations of the input signal that are not subject to issues of mismatch or drift. The LTC2512-24 offers many
similarities to the LTC2508-32, with four pin-selectable decimation filters, as shown in Table 2. Figure 2 shows the amplitude response of the digital filter from DC to fO, the output data rate. Downsampling factors from four up to 32 are supported, resulting in a 3dB improvement in dynamic range for every 2x increase in downsampling factor. The LTC2512-24 has the same 22-bit composite code output, providing an almost ideal real time representation of the input signal. For the LTC2512-24, the output is always fully settled in 35 outputs samples. The LTC2508-32 is a great candidate
for seismology applications and oil and gas exploration, where the ADC must resolve extremely low level signals buried in noise. The wider bandwidth and flat passband of the LTC2512-24 may be more suitable for medical instruments such as EKGs, which benefit from the high dynamic range of the filtered output, while utilising the no latency output for real time information such as probe location. These ADCs are ideal for any precision application utilising a control loop that must react quickly to changes in the input signal which would not be immediately visible via the slower, filtered output.
Linear Technology
www.linear.com
18 SEPTEMBER 2017 | INSTRUMENTATION
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