Getting signals into an oscilloscope or digitizer without distorting them is a significant concern for instrument designers and users. The critical first point in an instrument is the input port. Oscilloscopes offer 50-Ω and 1-MΩ input terminations for both channels and trigger inputs. When should each be used?
A typical oscilloscope offers input ports terminated in either a 50-Ω, DC-coupled, or 1-MΩ AC- or DC-coupled, or ground (Figure 1).
Figure 1 The input coupling choices for a typical oscilloscope include 50-Ω DC, 1-MΩ AC or DC, and ground. Source: Art Pini
Input termination
The 50-Ω termination is intended for use with 50-Ω sources connected to the oscilloscope using a 50-Ω coaxial cable. The 50-Ω input properly terminates the coaxial cable, preventing reflections and associated signal losses.
The 50-Ω input termination is also used with certain probes. The simplest probe, based on a 50-Ω termination, is the transmission line or low-capacitance probe (Figure 2).
Figure 2 The low capacitance probe is intended to probe low impedance sources like transmission lines. A 10:1 attenuation is achieved by making RIN equal 450 Ω, which results in a 10:1 attenuation of the probed signal. Source: Art Pini
The transmission line probe has a relatively wide bandwidth, typically ranging from 5 GHz or more. Its input impedance is low, and in the case of a 10:1 probe, it is only about 500 Ω. Most other active high-bandwidth probes also utilize the 50-Ω oscilloscope input termination.
The 1-MΩ input termination is intended to connect to 10:1 high-impedance passive probes as shown in Figure 3.
Figure 3 A simplified schematic for a 10:1 high impedance passive probe connected to an oscilloscope’s 1 MΩ input. Source: Art Pini
The high-impedance probe places a 9-MΩ resistor in series with the oscilloscope’s 1-MΩ input termination, forming a 10:1 attenuator. This passive probe has a DC input resistance of 10 MΩ. The compensation capacitor (Ccomp) is adjusted so that the time constants Rin*Cin are the same as Ro*(Co+Ccomp), forming an all-pass filter offering a relatively square low-frequency pulse response. The 1-MΩ termination also serves as a high-impedance input for low-frequency measurements, where reflections are not an issue.
50-Ω vs 1-MΩ inputs
The two input terminations have significant differences. Consider the Teledyne LeCroy HDO 6104B, a 1 GHz mid-range oscilloscope, as an example (Table 1).
Input Termination | Bandwidth | Coupling | Vertical Range (V/div) | Offset Range | Maximum Input |
50 Ω | 1 GHz | DC, GND | 1 mV to 1 V | ± 1.6 V to ±10 V | 5 Vrms, ± 10 V p-p |
1 MΩ | 500 MHz | AC, DC, GND | 1 m V to 10 V | ± 1.6 V to ± 400 V | 400 V(DC + Peak AC<10 kHz) |
Table 1 The characteristics of the input terminations are quite different. Source: Art Pini
The bandwidth of the 50-Ω termination is usually much greater than that of the 1-MΩ termination. In this example, it is 1 GHz. The oscilloscope’s bandwidth is generally specified for the 50-Ω termination. The 50-Ω input has a more limited input voltage range than the 1-MΩ input. The maximum voltage range of the 50-Ω termination is power-limited to 5 Vrms by the ½-watt rating of the input resistor. The 1-MΩ has a maximum voltage rating of 400 V (DC+AC peak <10 kHz). The 50-Ω input is only available in DC-coupled mode, whereas the 1-MΩ termination is available in both AC and DC-coupled modes. Finally, the offset range of the 1-MΩ input extends up to ±400 V while the 50-Ω offset range is ±10 V.
DC or AC input coupling
DC coupling applies to the entire frequency spectrum of the signal’s frequency components from DC to the full rated bandwidth of the instrument’s specified input. AC coupling filters out the DC by placing a blocking capacitor in series with the oscilloscope input. The series capacitor acts like a high-pass filter.
In most oscilloscopes, the AC-coupled input has a lower cutoff frequency of about 10 Hz. AC-coupling the 50-Ω termination would require about 20,000 times larger capacitors to achieve the same 10-Hz lower cutoff frequency, so it is not done. This ability to separate a signal’s AC and DC components is utilized in applications such as measuring ripple voltage at the power supply’s output. The AC coupling blocks the power supply’s DC output while passing the ripple voltage. Figure 4 compares an AC- and a DC-coupled waveform with an offset voltage.
Figure 4 A 1 MHz, 381 mVpp, signal with a 100-mV DC offset is acquired using both DC (top trace) and AC (lower trace) coupling. Source: Art Pini
The upper trace shows the DC-coupled waveform. Note that the DC offset shifts the AC component of the input signal upward while the AC signal, shown in the lower trace, has a zero mean value. The AC coupling has removed the DC offset.
The peak-to-peak amplitude is measured using measurement parameter P1 as 381 mV. They are identical because the offset of the DC-coupled signal is canceled by the subtraction operation used in calculating the peak-to-peak value.
The DC-coupled signal has a DC offset of 100 mV, measured by the mean measurement parameters P2. The AC-coupled signal has the same peak-to-peak amplitude (P5) but a mean(P6) of near-zero V. The AC-coupled signal’s RMS amplitude (P3) reads 167.5 V because it includes the RMS value of the DC offset. The RMS value of the AC-coupled signal (P7) reads 134.4 mV because the mean value is zero. The DC-coupled signal’s standard deviation (sdev) (P4) is identical to the RMS values of the AC-coupled signal. Since the standard deviation calculation subtracts the mean value of the signal from the instantaneous value before computing RMS, the standard deviation is sometimes referred to as the AC RMS value.
Trigger input termination and coupling
The trigger input is another oscilloscope input that must be considered as it affects the instrument’s triggering. It is derived from one of the input channels, the external trigger input, or the power line. The trigger coupling for any of the inputs other than line is one of four possibilities: AC, DC, low-frequency reject (LF REJ), or high-frequency reject (HF REJ) (Figure 5).
Figure 5 The trigger input coupling selections include two bandwidth-limited modes (LF and HF REJ) and AC- and DC-coupling. Source: Art Pini
The AC and DC coupling perform as the AC and DC input coupling selections. LF REJ is an AC coupling mode with a high-pass filter in series with the trigger input. HF REJ is DC coupled with a low-pass filter in series with the trigger input. The cutoff frequencies of high-pass and low-pass filters are usually about 50 kHz. The LF and HF REJ coupling modes are usually used for noisy trigger signals, which might be encountered when testing switched-mode power supplies.
If the trigger input source is one of the input channels, then the trigger input inherits the termination impedance of the input channel. If the external trigger input is used, the input impedance can be selected (Figure 6).
Figure 6 The termination of the external trigger input includes both 50-Ω and 1-MΩ DC, along with a 1:1 and a 10:1 attenuator. Source: Art Pini
The termination is either 50 Ω or 1 MΩ. The external trigger is DC-coupled from the physical input to termination. The trigger coupling selection sets the coupling between the termination and the trigger.
Selecting input terminations when using a probe
Most modern oscilloscopes have intelligent probe interfaces that sense the probe’s presence and read its characteristics. The instrument adjusts the input termination and attenuation to match the probe’s requirements. For classical passive probes, simpler 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 sense pin or an intelligent interface, then the attenuation setting of the input channel must be done manually.
50-Ω termination workarounds
The 50-Ω termination offers the highest bandwidth and is used with signal sources connected via 50-Ω coaxial cables or active probes that expect a 50-Ω termination. Serial in-line attenuations can be used to increase the voltage range of the 50-Ω input. AC coupling of the 50-Ω input can be accomplished using an external blocking capacitor. The lower frequency cutoff will be a function of the block’s capacitance.
Other traditional terminating impedances can be adapted to the 50-Ω termination by using an external in-line impedance pad. This is particularly common in applications such as video, where 75-Ω terminations are the standard. If an impedance pad is used, the pad’s attenuation has to be manually entered into the input channel setup.
1-MΩ termination workarounds
The 1-MΩ termination provides a high input impedance, which reduces circuit loading. It offers the highest voltage and offset ranges, but its bandwidth is restricted to 500 MHz or less. Care should be exercised when using it to measure low-impedance sources with frequencies greater than 40 to 50 MHz to avoid reflections, which will manifest themselves as ringing (Figure 7).
Figure 7 Measuring a low-impedance source using a 1-MΩ input can result in reflections that look like ringing (upper trace). Using a 50-Ω termination (lower trace) does not show the problem. Source: Art Pini
If you must use a 1-MΩ input, reflections can be reduced by soldering a 50-Ω resistor to the low-impedance source and connecting the 1-MΩ input to the resistor. This will help reduce reflections from the high-impedance termination back to the source.
The rail probe is the best of all possible worlds
Given that a typical application of oscilloscopes is measuring power supply ripple, the DC-coupled input’s limited offset voltages and the AC-coupled inputs’ attenuation call for a unique solution. The rail probe is a solution to measuring ripple on power rails that offers a large built-in offset, low attenuation, and high DC input impedance. The rail probe’s built-in offset and low attenuation permit the rail voltage to be offset in the oscilloscope by its mean DC voltage with high oscilloscope vertical sensitivity, achieving a noise-free view of small signal variations. The high DC input impedance eliminates the loading of the DC rail.
The input termination and coupling are important when setting up a measurement. Keep in mind how they can affect the signal acquisition and subsequent analysis.
Arthur Pini is a technical support specialist and electrical engineer with over 50 years of experience in electronics test and measurement.
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