Maxim MAX12557 manual Applications Information, Using Transformer Coupling

Dual, 65Msps, 14-Bit, IF/Baseband ADC

MAX12557

Applications Information

Using Transformer Coupling

In general, the MAX12557 provides better SFDR and THD with fully differential input signals than single- ended input drive, especially for input frequencies above 125MHz. In differential input mode, even-order harmonics are lower as both inputs are balanced, and each of the ADC inputs only requires half the signal swing compared to single-ended input mode.

An RF transformer (Figure 9) provides an excellent solution to convert a single-ended input source signal to a fully differential signal, required by the MAX12557 for optimum performance. Connecting the center tap of the transformer to COM provides a VDD / 2 DC level shift to the input. Although a 1:1 transformer is shown, a step-up transformer can be selected to reduce the drive requirements. A reduced signal swing from the input driver, such as an op amp, can also improve the

			24.9Ω
			IN_P
			5.6pF
	0.1∝F		MAX12557
VIN	1	6
	T1
	5	2	COM_
	N.C.
	3	4	0.1∝F
	MINICIRCUITS
	TT1-6
	OR		24.9Ω
	T1-1T		IN_N
			5.6pF

overall distortion. The configuration of Figure 9 is good for frequencies up to Nyquist (fCLK / 2).

The circuit of Figure 10 converts a single-ended input signal to fully differential just as Figure 9. However, Figure 10 utilizes an additional transformer to improve the common-mode rejection allowing high-frequency signals beyond the Nyquist frequency. A set of 75Ω and 113Ω termination resistors provide an equivalent 50Ω termination to the signal source. The second set of termination resistors connects to COM_ providing the correct input common-mode voltage. Two 0Ω resistors in series with the analog inputs allow high IF input fre- quencies. These 0Ω resistors can be replaced with low- value resistors to limit the input bandwidth.

Single-Ended AC-Coupled Input Signal

Figure 11 shows an AC-coupled, single-ended input application. The MAX4108 provides high speed, high bandwidth, low noise, and low distortion to maintain the input signal integrity.

VIN	0.1∝F	0Ω
MAX4108		IN_P
		5.6pF
100Ω		24.9Ω
		MAX12557
		COM_
		0.1∝F
100Ω		24.9Ω
		IN_N
		5.6pF

Figure 9. Transformer-Coupled Input Drive for Input Frequencies	Figure 11. Single-Ended, AC-Coupled Input Drive
Up to Nyquist

							0Ω*
	0.1∝F						5.6pF
	1	6		1	6
VIN			75Ω				113Ω
	T1			T2
			1%				0.5%
	5	2	N.C.	5	2	N.C.
	N.C.			N.C.
			75Ω				0.1∝F
	3	4		3	4		113Ω
	MINICIRCUITS		1%	MINICIRCUITS			0.5%
							0Ω*
	ADT1-1WT			ADT1-1WT
			*0Ω RESISTORS CAN BE REPLACED WITH				5.6pF

LOW-VALUE RESISTORS TO LIMIT THE INPUT BANDWIDTH.

IN_P

MAX12557

COM_

IN_N

Figure 10. Transformer-Coupled Input Drive for Input Frequencies beyond Nyquist

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