Key Elements in Selecting High-speed, High-performance Digitizer

Today, I would like to introduce an analog input/output interface board that may seem a little unfamiliar in the machine vision industry. Sensors that are generally readily used around measuring temperature, pressure, flowrate, vibration, etc. displays analog signals, and actuators used for control are mostly operated by analog signals. However, in a PC used for measurement, inspection, etc., an interface board is required to change the analog signal to digital signal in order to process the above-mentioned analog signals. For this, I would like to introduce high-speed, high-performance digital board from our company that is ideal to these applications, while explaining important factors in the selection of digitizer.

AVALDATA’s analog-to-digital Express Converter series has been developed for measurement inspection and communication devices requiring a sampling rate of 200MSps to 3GSps. Combination of high-speed ADC(Analog to Digital Conversion) and our PCI Express IP core technology enables the transfer of high-speed sampling data to PCs without missing data. To some extent, we also provide FPGA customization to customers.

1. Comparison between Oscilloscope and Digitizer

Above all, in order to help you understand the digitizer, we need to look at the features, advantages, and disadvantages of digitizer, in comparison with oscilloscopes that are commonly used in industrial measurement.

Figure 1. Oscilloscope

Figure 2. Digitizer

Oscilloscopes(Figure 1) are mainly used for trouble-shooting of electronic circuits, and there are also high-speed models with a sampling rate exceeding 100GSps in line with the recent rapid acceleration of electronic circuits. Most of them have a low resolution of 8bits and is used to check waveform information at the time of problem occurring, however, it is not suitable for the applications that transfer data externally due to lack of an external interface.

Above image(Figure 2.) shows a PCI Express type product of AVALDATA that can be installed and used in a PC. The sampling rate is between 200MSps and 1GSps, however, it offers a high resolution of 12 to 16bits, which makes it suitable for signal analysis that requires high SNR. It also provides a high-speed interface, which enables continuous external transmission of data, which is used in many areas such as inspection, instrumentation, and video analysis devices.

2. Criteria in Selecting Digitizer

If a digitizer is selected based on the sampling rate and signal bandwidth, the signal bandwidth, which means the frequency range that the digitizer can enter, becomes an important point for determining the sampling rate, hence selecting the suitable board by checking the frequency of the signal the user want to measure. Below I would like to point out the relationship between sample rate and signal bandwidth.

Sampling Rate : 200MSps

Figure 3. Sampled waveform against input signal

The above graphs are a waveform with five different input signals (10MHz, 50MHz, 100MHz, 150MHz, 190MHz) sampled at 200MSps sample rate using our digitizer, and the signal bandwidth of the digitizer was set to DC~100Mhz.
In Case 1(10MHz) and Case5(190MHz), where the input relative to sampling rate corresponds to 1/20, the signal reproducibility is high, and the signal’s amplitude is also visible, and the wavelengths of these two input signals are consistent. In Case2(50MHz) corresponding to 1/4 and Case3(100MHz) corresponding to 1/2, the signal is less reproducible and the state of the original signal becomes difficult to see. In addition, if we increase the signal speed to 150MHz, the amplitude of the signal will be completely eliminated, resulting in a similar phenomenon to a 50MHz signal sampled.

3. Fast Fourier Transform

While the previous tests showed that 150Mhz and 50Mhz, 190Mhz and 10Mhz had the same results, the Fast Fourier Transform gives a more detailed understanding of the phenomenon. At a sample rate of 200Mh, if you set the input signal to 190Mhz, it shows a frequency at the front 10Mhz point that was not originally present. Such relationship is replicated such as the case like folding one-half the frequency of the sampling rate, part of the sampling frequency inward and outward, etc. These replications, a.k.a., aliasing phenomenon, are continuously taking place in logically high-frequency directions.

Figure 4. FFT Analysis

4. Key Performance of Digitzier (Explanation with APX-5360G3, 1.8GSps, 12bit )

Looking at the specifications of our Digitizer, APX-5360, it contains a low-pass filter which is attenuated by -3dB(approximately 30%) at 900Mhz, which is 1.8Gsps in sampling rate and 50% level of 1.8Gsps signal band. 3rd party’s digitizers are also equipped with low-pass filters or band-pass filters to attenuate Aliasing. Filters that prevent Aliasing are referred to as Anti-aliasing filters.

Anti-aliasing Filter ? 
Because the cut-off frequency of the Anti-aliasing Filter depends on the digitizer maker, it is necessary to select a digitizer in a wider band than the frequency of the signal you want to measure. For AVALDATA’s digitizer, the cut-off frequency of the Anti-aliasing filter and the point at -3dB are set to the Nyquist Frequency of each digitizer for use by a wide range of customers.

Table 1. APX-5360 Specification

5. Digitizer’s Noise-Performance Indicator

There are four performance indicators in the noise specifications of the digitizer.

1. SNR(Signal to Noise Ratio) is the ration of the input signal to other frequency components. However, high frequency components are excluded from noise. In ideal sine waves, high frequency components occur, but if the signal has any nonlinearity, high frequency is expressed as an integer multiple of the input signal since it breaks down into repetitive frequency and high frequency components. High frequency is excluded from the noise component because the input signal itself is believed to be a cause.
2. SINAD(Signal to Noise And Distortion Ratio) is a calculation of the ratio of input signals, including high frequencies, to other frequency components, and is always lower than SNR.
3. SFDR(Spurious Free Dynamic Range) refers to the ratio of the input signal to the next higher frequency and also includes the high frequency components that result from the input signal.
4. ENOB(Effective number of bits) means an indicator that indicates a valid number of bits and can be calculated as ENOB by SINAD.
   ex) Formula   ENOB = (SINAD – 1.76) / 6.02

The specifications given in Table-1 show that APX-5360 has a resolution of 12bits and SNR value of 56.3dB.
Assuming that the value of the input signal is 1, the total noise component is about 0.001531

6. Resolution & SNR

The use of high resolution digitizer allows the acquisition of high SNR. There is no problem if the amplitude of the signal to be measured is large, however, when measuring a fine signal of -40dB (approx. 1%) of the digitizer, even if it is a digitizer with a performance of -70dB at full scale input, the SNR is -30dB as being compared with the input signal. In general, it is known to be difficult to determine signals and noise if the level of the signal is not greater than 5 to 6dB being compared to noise, so it is advantageous to analyze the frequency by the digitizer having a higher SNR if the level of the signal to be measured is smaller.

The intensity of the spectrum is expressed in unit of dB. On the digitizer, the value is expressed as 1/100 of a -40dB and 1/10000 of a -80dB as the number indicates the amount of noise compared to the input signal, and the larger the number, the better the performance. In the case of dB, the individual values are not meaningful as relative values are and only meaningful from comparison with other values.

Figure 5. Frequency Spectrum

Decibel(dB) indication

The solution that employs the PCI Express interface will certainly continue to be fast-paced. PCI Express is expected to evolve from the current 2.0 to 3.0 and 4.0, and memory from DDR4 to DDR5, with digitizer boards also likely to release products with a 5Gsps or higher ADC, and product(APX-5056, GO to check specification) with a four-channel sampling capability for 500Mhz and 16 bits has been launched recently. In the future, when considering high-performance digitizer to use, it is necessary to select the product in consideration of the synchronization error between signals, usage of the external clock and the need for signal processing by the FPGA.