Getting indicators into an oscilloscope or digitizer with out distorting them is a big concern for instrument designers and customers. The vital first level in an instrument is the enter port. Oscilloscopes provide 50-Ω and 1-MΩ enter terminations for each channels and set off inputs. When ought to every be used?

A typical oscilloscope affords enter ports terminated in both a 50-Ω, DC-coupled, or 1-MΩ AC- or DC-coupled, or floor (Determine 1).

Determine 1 The enter coupling decisions for a typical oscilloscope embody 50-Ω DC, 1-MΩ AC or DC, and floor. Supply: Artwork Pini

Enter termination

The 50-Ω termination is meant to be used with 50-Ω sources related to the oscilloscope utilizing a 50-Ω coaxial cable. The 50-Ω enter correctly terminates the coaxial cable, stopping reflections and related sign losses.

The 50-Ω enter termination can be used with sure probes. The only probe, primarily based on a 50-Ω termination, is the transmission line or low-capacitance probe (Determine 2).

Determine 2 The low capacitance probe is meant to probe low impedance sources like transmission traces. A ten:1 attenuation is achieved by making R_IN equal 450 Ω, which leads to a ten:1 attenuation of the probed sign. Supply: Artwork Pini

The transmission line probe has a comparatively broad bandwidth, usually starting from 5 GHz or extra. Its enter impedance is low, and within the case of a ten:1 probe, it is just about 500 Ω. Most different energetic high-bandwidth probes additionally make the most of the 50-Ω oscilloscope enter termination.

The 1-MΩ enter termination is meant to connect with 10:1 high-impedance passive probes as proven in Determine 3.

Determine 3 A simplified schematic for a ten:1 excessive impedance passive probe related to an oscilloscope’s 1 MΩ enter. Supply: Artwork Pini

The high-impedance probe locations a 9-MΩ resistor in sequence with the oscilloscope’s 1-MΩ enter termination, forming a ten:1 attenuator. This passive probe has a DC enter resistance of 10 MΩ. The compensation capacitor (C_comp) is adjusted in order that the time constants R_in*C_in are the identical as R_o*(C_o+C_comp), forming an all-pass filter providing a comparatively sq. low-frequency pulse response. The 1-MΩ termination additionally serves as a high-impedance enter for low-frequency measurements, the place reflections are usually not a difficulty.

50-Ω vs 1-MΩ inputs

The 2 enter terminations have important variations. Take into account the Teledyne LeCroy HDO 6104B, a 1 GHz mid-range oscilloscope, for instance (Desk 1).

Desk 1 The traits of the enter terminations are fairly totally different. Supply: Artwork Pini

The bandwidth of the 50-Ω termination is normally a lot better than that of the 1-MΩ termination. On this instance, it’s 1 GHz. The oscilloscope’s bandwidth is mostly specified for the 50-Ω termination. The 50-Ω enter has a extra restricted enter voltage vary than the 1-MΩ enter. The utmost voltage vary of the 50-Ω termination is power-limited to five Vrms by the ½-watt score of the enter resistor. The 1-MΩ has a most voltage score of 400 V (DC+AC peak <10 kHz). The 50-Ω enter is barely out there in DC-coupled mode, whereas the 1-MΩ termination is accessible in each AC and DC-coupled modes. Lastly, the offset vary of the 1-MΩ enter extends as much as ±400 V whereas the 50-Ω offset vary is ±10 V.  

DC or AC enter coupling

DC coupling applies to all the frequency spectrum of the sign’s frequency elements from DC to the total rated bandwidth of the instrument’s specified enter. AC coupling filters out the DC by putting a blocking capacitor in sequence with the oscilloscope enter. The sequence capacitor acts like a high-pass filter.

In most oscilloscopes, the AC-coupled enter has a decrease cutoff frequency of about 10 Hz. AC-coupling the 50-Ω termination would require about 20,000 occasions bigger capacitors to attain the identical 10-Hz decrease cutoff frequency, so it isn’t completed. This capability to separate a sign’s AC and DC elements is utilized in purposes reminiscent of measuring ripple voltage on the energy provide’s output. The AC coupling blocks the ability provide’s DC output whereas passing the ripple voltage. Determine 4 compares an AC- and a DC-coupled waveform with an offset voltage.

Determine 4 A 1 MHz, 381 mVpp, sign with a 100-mV DC offset is acquired utilizing each DC (prime hint) and AC (decrease hint) coupling. Supply: Artwork Pini

The higher hint exhibits the DC-coupled waveform. Notice that the DC offset shifts the AC part of the enter sign upward whereas the AC sign, proven within the decrease hint, has a zero imply worth. The AC coupling has eliminated the DC offset.

The height-to-peak amplitude is measured utilizing measurement parameter P1 as 381 mV. They’re an identical as a result of the offset of the DC-coupled sign is canceled by the subtraction operation utilized in calculating the peak-to-peak worth.

The DC-coupled sign has a DC offset of 100 mV, measured by the imply measurement parameters P2. The AC-coupled sign has the identical peak-to-peak amplitude (P5) however a imply(P6) of near-zero V. The AC-coupled sign’s RMS amplitude (P3) reads 167.5 V as a result of it contains the RMS worth of the DC offset. The RMS worth of the AC-coupled sign (P7) reads 134.4 mV as a result of the imply worth is zero. The DC-coupled sign’s commonplace deviation (sdev) (P4) is an identical to the RMS values of the AC-coupled sign. Since the usual deviation calculation subtracts the imply worth of the sign from the instantaneous worth earlier than computing RMS, the commonplace deviation is typically known as the AC RMS worth.

Set off enter termination and coupling

The set off enter is one other oscilloscope enter that have to be thought-about because it impacts the instrument’s triggering. It’s derived from one of many enter channels, the exterior set off enter, or the ability line. The set off coupling for any of the inputs aside from line is one in all 4 prospects: AC, DC, low-frequency reject (LF REJ), or high-frequency reject (HF REJ) (Determine 5).

Determine 5 The set off enter coupling choices embody two bandwidth-limited modes (LF and HF REJ) and AC- and DC-coupling. Supply: Artwork Pini

The AC and DC coupling carry out because the AC and DC enter coupling choices. LF REJ is an AC coupling mode with a high-pass filter in sequence with the set off enter. HF REJ is DC coupled with a low-pass filter in sequence with the set off enter. The cutoff frequencies of high-pass and low-pass filters are normally about 50 kHz. The LF and HF REJ coupling modes are normally used for noisy set off indicators, which could be encountered when testing switched-mode energy provides.

If the set off enter supply is likely one of the enter channels, then the set off enter inherits the termination impedance of the enter channel. If the exterior set off enter is used, the enter impedance might be chosen (Determine 6).

Determine 6 The termination of the exterior set off enter contains each 50-Ω and 1-MΩ DC, together with a 1:1 and a ten:1 attenuator. Supply: Artwork Pini

The termination is both 50 Ω or 1 MΩ. The exterior set off is DC-coupled from the bodily enter to termination. The set off coupling choice units the coupling between the termination and the set off.

Deciding on enter terminations when utilizing a probe

Most fashionable oscilloscopes have clever probe interfaces that sense the probe’s presence and browse its traits. The instrument adjusts the enter termination and attenuation to match the probe’s necessities. For classical passive probes, less complicated probe interfaces sense the probe’s sense pin to detect its presence and attenuation and set the instrument coupling and attenuation to match the probe. If the passive probe lacks a way pin or an clever interface, then the attenuation setting of the enter channel have to be completed manually.

50-Ω termination workarounds

The 50-Ω termination affords the very best bandwidth and is used with sign sources related through 50-Ω coaxial cables or energetic probes that count on a 50-Ω termination. Serial in-line attenuations can be utilized to extend the voltage vary of the 50-Ω enter. AC coupling of the 50-Ω enter might be achieved utilizing an exterior blocking capacitor. The decrease frequency cutoff will probably be a perform of the block’s capacitance.

Different conventional terminating impedances might be tailored to the 50-Ω termination by utilizing an exterior in-line impedance pad. That is notably widespread in purposes reminiscent of video, the place 75-Ω terminations are the usual. If an impedance pad is used, the pad’s attenuation needs to be manually entered into the enter channel setup.

1-MΩ termination workarounds

The 1-MΩ termination gives a excessive enter impedance, which reduces circuit loading. It affords the very best voltage and offset ranges, however its bandwidth is restricted to 500 MHz or much less. Care must be exercised when utilizing it to measure low-impedance sources with frequencies better than 40 to 50 MHz to keep away from reflections, which can manifest themselves as ringing (Determine 7).

Determine 7 Measuring a low-impedance supply utilizing a 1-MΩ enter can lead to reflections that appear to be ringing (higher hint). Utilizing a 50-Ω termination (decrease hint) doesn’t present the issue. Supply: Artwork Pini

If you happen to should use a 1-MΩ enter, reflections might be lowered by soldering a 50-Ω resistor to the low-impedance supply and connecting the 1-MΩ enter to the resistor. This can assist cut back reflections from the high-impedance termination again to the supply.

The rail probe is the very best of all attainable worlds

Given {that a} typical utility of oscilloscopes is measuring energy provide ripple, the DC-coupled enter’s restricted offset voltages and the AC-coupled inputs’ attenuation name for a singular answer. The rail probe is an answer to measuring ripple on energy rails that gives a big built-in offset, low attenuation, and excessive DC enter impedance. The rail probe’s built-in offset and low attenuation allow the rail voltage to be offset within the oscilloscope by its imply DC voltage with excessive oscilloscope vertical sensitivity, attaining a noise-free view of small sign variations. The excessive DC enter impedance eliminates the loading of the DC rail.

The enter termination and coupling are vital when organising a measurement. Take into account how they’ll have an effect on the sign acquisition and subsequent evaluation.

Arthur Pini is a technical assist specialist and electrical engineer with over 50 years of expertise in electronics check and measurement.

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