The art of analog design is often hidden in the specifications. Behind specifications such as bandwidth and effective number of bits (ENOB) are characteristics such as passband flatness, step response, linearity, and noise. PXIe-5164 analog design characteristics include a wide dynamic range analog-to-digital converter (ADC) and analog front end, robust high-voltage design with no loss in high-resolution performance, and the ability to use digital signal processing (DSP) to stabilize and equalize the magnitude and phase responses.
Wide Dynamic Range ADC and Front End
The PXIe-5164 is designed for measurement accuracy without sacrificing range
The PXIe-5164 uses a Texas Instruments ADS54J40, an ADC remarkable for its low noise spectral density specification of 155.9 dBFS/Hz. To maintain this low noise floor in an oscilloscope, you need to depart from the usual circuit arrangement. Figure 2(a) shows the block diagram of a typical oscilloscope where the 50 Ohm mode is accomplished by simply connecting a 50 Ohm resistor in parallel with the front panel input. The signal passes through the 1 MOhm buffer which is optimized for high input impedance and high voltage. By necessity, less design emphasis is placed on the noise and distortion performance in the 1 MOhm buffer. Figure 2(b) shows the approach taken in the PXIe-5164 oscilloscope where a dedicated 50 Ohm path bypasses the 1 MOhm section and connects directly to the low-impedance amplifier section. The performance limitations of the 1 MOhm stage are thus avoided. The low-impedance amplifier in Figure 2(b) is a single-stage silicon germanium ADC driver with variable feedback. Using one amplifier rather than several (several stages being characteristic of most oscilloscope designs) eliminates the noise and distortion contributions of the multiple stages. Also, gain control in this single-stage circuit is accomplished by changing the degree of feedback rather than by varying the degree of input attenuation (again, the latter being characteristic of the usual approach). So, the highest level of performance is obtained in the 50 Ohm mode.
Figure 2. The PXIe-5164 oscilloscope has a signal path (b) optimized for measurement accuracy by bypassing the 1 MegOhm buffer on the 50 Ohm path and having only a single gain stage, as opposed to a usual oscilloscope signal path (a) that is optimized for high impedance and voltage.
You can most clearly see the superior dynamic range of the PXIe-5164 in side-by-side measurements with a popular 8-bit box oscilloscope. Figure 3 shows time domain plots of a one-time-event communications signal riding on a digital pulse taken with a box scope and a PXIe-5164 oscilloscope. The communications signal amplitude is one percent of the pulse amplitude. Both instruments were set for 50 Ohms on their 2 Vpp ranges. To improve signal-to-noise ratio, the bandwidth of the box scope was reduced to its 250 MHz setting and its acquisition mode was set to high resolution. The communications signal is nearly indiscernible in the data captured with the 8-bit box scope but easily recognizable and decodable in the data captured by the PXIe-5164 oscilloscope.
Figure 3. Compare the time domain plots of a popular 8-bit box oscilloscope with the PXIe-5164 oscilloscope with both sampling an identical one-time-event communications signal riding on a digital pulse. The communications signal is nearly indiscernible on the 8-bit box scope but easily recognized and decodable on the PXIe-5164.
Figure 4 shows spectral results of the same box scope and the PXIe-5164. Input is a 101 MHz sine tone at 11 dBm. Both oscilloscopes were set to 50 Ohms, 2 Vpp range, and full bandwidth with the fast Fourier transform (FFT) number of points set to one million. The noise floor of the PXIe-5164 oscilloscope is an astonishing 22 dB lower. If the size of the FFT of PXIe-5164 data is reduced to 200,000 points so that both FFTs have the same 2.5 KHz bin width (the box instrument samples at 5 Gs/s versus 1 Gs/s for the PXIe-5164), the noise floor difference is still an impressive 16 dB. The PXIe-5164 oscilloscope also has the better harmonic distortion performance.
Figure 4. By comparing the spectral results of a popular 8-bit box oscilloscope with the PXIe-5164 oscilloscope, you can see the noise floor of the PXIe-5164 oscilloscope is an astonishing 22 dB lower.
Robust High-Voltage Design With No Loss in High-Resolution Performance
The simplicity of the 50 Ohm signal path in the conventional oscilloscope affords a high degree of overload protection but compromises performance because the 1 MOhm buffer is included. The PXIe-5164 oscilloscope signal path design overcomes this problem by having a special series protection circuit in the directly coupled 50 Ohm path (the block labeled “protection” in Figure 2[b]). A high degree of protection is obtained without compromising the 50 Ohm performance.
Most oscilloscopes implement high voltage ranges through one or more compensated voltage divider that you switch in and out using bulky electromechanical relays, as Figure 2(a) shows. The relays consume valuable board area that you could use to implement a variety of other necessary oscilloscope functions. The PXIe-5164 oscilloscope avoids this by using a patent-pending technique where all switching is done at low voltage. The PXIe-5164 can measure voltage swings up to +/-50 V on offset voltages up to +/-250 V with no need for problematic electromechanical relays.
DSP Stabilizes and Equalizes the Magnitude and Phase Responses
Variability is inevitable in the frequency and step response of an oscilloscope’s analog front end. Digital filtering can offer a considerable improvement in range-to-range, channel-to-channel, and even unit-to-unit variability. Moreover, you can use digital filtering to adjust the response for other characteristics such as passband flatness and linear phase response. In the PXIe-5164 oscilloscope, a 16-tap finite impulse response (FIR) filter is in the FPGA and in line with the ADC data stream to realize these advantages. The filter parameters are determined during factory or external calibration and transferred from EEPROM to the FIR filter depending on range, impedance, and filter setting. The calibration procedure mandates a very flat-with-frequency power meter, hence a guaranteed and traceable flat frequency response is transferred to the PXIe-5164 during calibration. Flatness at +/- 0.5 dB to 330 MHz is guaranteed. Figure 5 shows a typical PXIe-5164 oscilloscope measured response where the deviation from 0 dB is less than 0.022 dB for all ranges and channels. Figure 6 shows a measured step response where symmetry in the waveform indicates the wanted linear phase characteristic.
Figure 5. This graph shows the frequency response of a typical PXIe-5164 oscilloscope on the 50 Ohm path up to the full bandwidth (400 MHz). The response of all ranges and channels of one module are superimposed and the maximum deviation in the passband is 0.022.
Figure 6. PXIe-5164 oscilloscope 50 Ohm step response, full bandwidth (400 MHz). Symmetry in the step response is the result of the minimum phase characteristic imposed by the in-line FIR filter.