Ti's DAC34H84 is a 4-channel, 16-bit, 1250 msps DAC. The reason is that it is a typical high-speed number of modulus, has an isolated input and DAC clock field input FIFO, interpolated digital module, fine frequency resolution digital orthogonal modulation, analog orthogonal modulator correction and SIN (x) / x calibration (see Figure 1). This article will be introduced one by one to these features and functions.
The first digital module is an interpolation module that is responsible for increasing the sampling rate of the DAC internal digital signal. In general, the interpolation is achieved by adding steps using twice sampling rate. This work is accomplished using zero between the input sampling points, which produces two signals in FIF and FIN - FIF. After a digital low pass filter, the second signal located in the FIN-FIF is removed, only the signal is left in the FIF. The reason why the interpolation is used is related to the zero-order maintenance output structure used by most high speed DACs. With zero step hold, DAC performs corresponding settings according to the digital sampling of the clock cycle, then holds, until the clock cycle and the next output sampling end. This produces an "upper-stairclar" output, and its frequency response is as shown in equation 1:
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Figure 1 DAC34H84 function structure diagram
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Among them, FIF is an analog output frequency, while FS is a sampling rate. This response has a low-pass effect (see Figure 2), and the loss of f = fs / 2 is ~ 3.5 dB, and is zero at the fs multiple. Although the DAC outputs there will be signal images when the N * FS +/-fif is, but the image amplitude of the NYQUIST area is much lower than the signal at the FIF, thereby having a lower signal to noise ratio (SNR) And may have a significant amplitude drop. This limits most applications to the output signal frequency below FS / 2. In addition, the interval between the signals and the FS - FIF images are reduced as the FIF is close to FS / 2, so that the DAC output analog filter (action is to remove FS - FIF extra image) is difficult to establish, eventually Most applications are limited to FS / 3 or less.
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Figure 2 DAC output spectrum without interpolation module
With the DAC interpolation module, add the DAC internal sampling rate, just let the DAC's digital interface rate fin is high enough to allow signal bandwidth transmission, and only increase the small amount of additional bandwidth can have interpolation filter transition band (Fin when real signals). 1882.5 * BW, the complex signal is fin > 1.25 * bw). Using interpolation increases the sampling rate, the signal can be easily located below FS / 2.
Another advantage of increasing the sampling rate is that the digital mixing can increase the output IF to a higher frequency. For example, use 2X interpolation, the output frequency can be higher than the FIN / 2, and this result cannot be obtained if the interpolation is not used (see Figure 3). In general, the complex input signal uses a compounder to avoid generating an image during the mixing process. The mixing output can be a real IF signal or a complex IF signal, which is valid after analog IQ modulator DAC.
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Figure 3 2X interpolated DAC output spectrum
The DAC output is used to simulate the orthogonal modulator (AQM), highlighting another useful digital characteristic-orthogonal modulator correction module with high speed DAC. The module is responsible for correcting the gain, phase, and offset imbalance of an orthogonal modulator, thereby improving AQM sideband suppression and LO feedback.
Finally, the digital FIR filter is located at the end of the digital signal chain, which is responsible for compensating for the SIN (X) / X high and low frequency regular attenuation of the first Nyquist region. In DAC34H84 implementation, the filter can provide compensation for up to 0.4 * FDAC, and the error is less than 0.03 dB.
As described herein, such as DAC34H84, high-speed DAC has a large number of digital features. These features make the system implementation simple and easy by reducing the data rate and improving the output signal characteristics. , Reading the full text, the technology area
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