![]() Without the VGA, the 10-bit ADC provides a dynamic range of about 60 dB. The Analog Devices Circuit Note presents an example for putting a VGA before the ADC. This will effectively extend the dynamic range of the cascade of the VGA and ADC. The gain of the VGA will be dynamically adjusted based on the amplitude of the input signal. For example, as shown in Figure 4, a receiver may employ a VGA before analog-to-digital conversion. In general, when the input signal has a wide dynamic range, we can employ a low-noise variable gain amplifier (VGA) before the ADC. Then, if necessary, in the following stages of the system, we can take into account that an extra multiplication by four was applied to the input. To circumvent this reduction in the effective number of bits of the ADC, we can simply multiply the weak signal by a factor of four and adjust its maximum to the reference voltage of the ADC. Effectively, the number of bits of the ADC is reduced from four in Figure 1 to two in Figure 2. And the ADC is equivalent to a two-bit ADC with a reference voltage of 4LSB. In this case, the levels greater than or equal to 4LSB are not utilized at all. This is due to the fact that the second experiment employs a smaller number of levels to approximate the analog signal. However, in Figure 2, we can hardly recognize that the red curve approximates a part of a sine waveform. In Figure 1, it’s almost easy to see that the red curve is a staircase approximation of $$x_1(t)$$. For the latter case, we are using only four discrete levels to digitize the analog signal. However, assume that, in certain geographic locations, the received signal is weak and the ADC must digitize $$x_2(t)$$ as illustrated in Figure 2. That’s why the reference voltage of the ADC is chosen to be equal to the maximum value of the input analog signal which is 2 volts. During the normal operation of the receiver, the ADC should digitize $$x_1(t)$$ which was depicted in Figure 1. To have a better insight, assume that this ADC is utilized in a hypothetical receiver. You may say that Figure 2 shows the expected functionality for our ADC and there is nothing wrong with it. As shown in Figure 2, the maximum of the input signal, which is 4LSB, is considerably smaller than the reference voltage of the ADC. Hence, in terms of decibels, the dynamic range of the ADC will be The maximum value is $$2^N-1$$ times the LSB value. The minimum value that can be detected is one least significant bit ( LSB). For an ADC, the dynamic range is related to the number of bits that are used to digitize the analog signal. The dynamic range is defined as the ratio between the largest and smallest values that the ADC can reliably measure. Then, we’ll see that to efficiently utilize the dynamic range of an ADC, the amplitude of the input signal should be adjusted to the reference voltage of the ADC. This article will review the definition of dynamic range. This specification becomes particularly important in applications such as wireless communications and instrumentation. This article will discuss the dynamic range specification of an ADC.ĭynamic Range (DR) is a common performance metric for analog-to-digital converters (ADCs). ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |