The best choice of input network to optimize the d

2022-07-31
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Correctly select the input network, optimize the dynamic performance and gain flatness of high-speed ADC

Abstract: this application note discusses how to select appropriate transformers and passive components, and obtain a wide gain flatness of input frequency response without sacrificing the dynamic performance of high-speed ADC

for analog-to-digital converters (ADCs) with high if, correct selection of board level components is a necessary condition to meet high dynamic performance and wide gain flatness. This technical document introduces how to select the input network and simplify the design of single ended to differential signal conversion with the help of broadband transformer, termination resistance and filter capacitor

this paper takes max1449 as an example to explain and analyze, and gives two possible input configurations. Figure 1 shows a typical AC coupled, single ended to differential conversion design. The design uses a wide-band transformer (such as tt-kk81 (200MHz) of ircuits, which has good plasticity under static load of mini-c), and the primary side is terminated with 50 resistors and 25/22pf filter network. In this configuration, a single ended input signal with a source impedance of 50 is converted into a differential signal through a transformer. 50 primary side termination can well match the signal source with the transformer. However, this also means that there is a mismatch between the primary and secondary sides of the transformer. The equivalent resistance of the primary side is 25, but there is a large impedance mismatch in the secondary side. This is because the 20K input resistance of ADC is connected in parallel with the 22pf capacitor. This will affect the frequency response characteristics of the input network, and ultimately affect the frequency response characteristics of the converter. The nominal leakage inductance of the transformer is 25nh to 100nh. Combined with the input filter capacitor of 22pf, this will generate a resonant frequency:

110mhz to 215mhz will generate interference spikes in this frequency band

figure 1

Figure 2 describes a similar AC coupling configuration, but it uses a broadband transformer with better performance with primary side termination, and develops innovative and sustainable adhesive solutions with industry partners (such as adtwt (800MHz) of mini circuits) and 25/10pf filter network. Although the impedance of adtwt is 75, the low leakage inductance increases the -1db frequency point to 400MHz, while the -1db frequency point of tt-kk81 is only 50MHz

figure 2

Figure 3 shows the comparison results of two termination architectures supplemented by transformer and filter network devices. From the figure, we can see the obvious performance improvement. The input bandwidth (blue line) of tt-kk81 transformer has a gain fluctuation of about 0.5dB between 90mhz and 110Mhz, while the input bandwidth (purple line) of adtwt transformer maintains a gain fluctuation of 0.1dB within 300MHz. Dynamic range (adtwt transformer, 50

figure 3.

primary side termination, input filter capacitance at InP and inn is 10PF) still has 58.4db SNR when fin=50mhz. Although Figure 3 only shows the input frequency of 80MHz to 260MHz (adtwt), the laboratory test results show that the input frequency can exceed the 8th Order Nyquist frequency within the range of 0.1dB gain fluctuation

improving the secondary impedance matching of the transformer can further improve the gain flatness. One method is to use secondary side termination instead of primary side termination. This method will be discussed in other application notes. This scheme is based on max1122/23/24 series recently launched by Maxim to design and analyze the input network. Please refer to the following application note links for details on primary and secondary side terminations

references max1448evkit datasheet, Rev1, 7/2001, Maxim integrated products, Sunnyvale, ca

max1449 datasheet, Rev0, 10/2000, Eucommia glue devices of Maxim integrated produc are in the construction stage T, Sunnyvale, ca

notes: secondary side transformer termination improves the gain flatness of high-speed ADC

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