Driving the future of technology : Original solution in measurement and control

 Lecture for measurement

2020/5/20 posted

Measurement of Small AC Signals and Low Noise Superimposed DC Signal

Here we introduce the best measuring method and the most suitable pre-amplifiers for measurement of superimposed small AC signals or ultra-low noise signals on DC power signal

Two key points we need to consider when measuring small AC signals superimposed on large DC voltage in order to achieve a high accuracy.

  •  Dynamic range of the measuring instruments
  •  Noise caused by the measuring instruments

 

Key points for measuring superimposed small AC signals and small noise on DC signal

1. Dynamic range of measuring instruments

It is effective to remove DC signals for measuring small AC signals or noise. For example, when we try to measure small AC signals of 1 mV superimposed on DC 10 V, if the DUT is directly connected to the measuring instrument, the measuring range should be set to 10V or higher. In order to detect 1 mV voltage change in 10V measuring range, it will require a measuring instrument with dynamic range of 80dB or greater (resolution of 14bit or higher). The larger the DC voltage is, the greater dynamic range of a measuring instrument is required, and obviously, it makes measuring a superimposed small AC signal more difficult. In addition, the dynamic range specification of the common measuring instruments we can find on market (such as oscilloscopes and etc.) is generally around 48dB. (8-bit of resolution.)

 

And the best solution for the measurement is to remove the DC voltage by AC coupling. The measuring range will be adjusted and matched to the level of small AC signals to be measured by AC coupling. That makes it possible to measure small AC signals on tens of DC volts with common measuring instruments. The required dynamic range to detect voltage fluctuation of 1mV in each measuring range is as shown in the table 1 below. For instance, when the measuring range setting is lowered to 100 mV, the dynamic range required to detect voltage fluctuation of 1 mV is 40 dB. Compared with before when the DC signal was removed, the dynamic range required is lowered by 40dB.

 

Table 1. Example of Required Dynamic Range to Detect Voltage Fluctuation of 1mV

Measurement Range Setting
(of measuring instrument)
Required Dynamic Range
(Resolution)
10 V 80 dB (14bit or higher)
1 V 60 dB (10bit or higher)
100 mV 40 dB (7bit or higher)

 

2. Noise caused by measuring instruments

When measuring small AC signals and noises, we need to consider the noise generated by the measuring instrument (measuring system) as well. For example, the noise level of most popular digital oscilloscopes are at several millivolts (mV), so it is impossible to measure small noise in the order of microvolts. (uV.) On such occasions, it is generally common to use a low noise amplifier in front of the measuring instrument to amplify signals. However, amplifiers specialized for low-noise characteristic have very low tolerance to overvoltage, when a signal with large DC voltage is applied to it, the amplifier is likely to break. Even for low noise amplifiers designated for AC signals, damage is likely to occur when excessive large amount of energy (current x voltage) is applied to the inputs. Furthermore, cautions are especially needed when outputs of power supplies or power amplifiers are connected to a low noise amplifier.

 

To prevent damage to the amplifiers caused by excessive inputs, a series circuit consists of a resistor and diode is usually used as an input protection circuit. (Figure 1.) When the DC voltage applied to the amplifier (Vin) is large, the protective resistance (R1) will increase to suppress the current applied (Iin) and protect the inputs of amplifier. However, when the protective resistance increases, the thermal noise of resistor itself also increases, and that will add up to the noise of amplifier. As we can see, the performance of input protection and the internal noise is in a trade-off relationship.

 

For example, when the input of the amplifier Vin is 40 V, the protective resistance R1 needs to be 4 kΩ or more to suppress the current Iin. However, the thermal noise of 4 kΩ resistor at room temperature is 8.1 nV/√Hz. Therefore, it is impossible to control the noise level any lower.

Despite of that, NF can provide a perfect solution with a pre-amplifier that has adopted an original current limiting protection circuit, which provides both inputs protecting performance and low noise characteristics.

Figure 1. Example of an Input Protection Circuit


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