LINEAR dn366f Data Handbook & Specifications

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Low Distortion, Low Noise Differential Amplifier Drives High Speed ADCs in Demanding Communications Transceivers Design Note 366 Cheng-Wei Pei

Introduction LT1993-x Features Today’s communications transceivers operate at much The LT1993-x is a fully differential input and output am- higher frequencies and wider bandwidths than those of plifier with up to 7GHz of gain-bandwidth product and an the past. Combining this with higher resolution require- impressive feature set. There are three fixed-gain options ments, transceiver design can become daunting. For with internal matched resistors: gain of 2 (6d B), gain of 4 engineers designing these systems, small noise and (12d B) and gain of 10 (20d B). The LT1993-x is DC-coupled, distortion budgets leave little flexibility when choosing precluding the need for DC blocking capacitors on the system components. inputs and outputs. The output common mode voltage is independently controlled with an external pin, allowing

The LT®1993-x is designed to meet the demanding require-

optimal bias conditions for the ADC inputs. The LT1993-x

ments of communications transceiver applications. It can

features two sets of differential outputs: a normal output

be used as a differential ADC driver or as a general-purpose

and a filtered output. The output filter eliminates additional

differential gain block. For single-ended systems, the

filtering in many applications, but if necessary, additional

LT1993-x can replace a transformer in performing single-

filtering can be achieved with a few external components.

ended to differential conversion without sacrificing noise

Figure 1 shows a block diagram of the LT1993-x.

or distortion performance.

High Speed ADC Driving

VEE One of the more challenging tasks in a modern commu-

–INA RG

nications transceiver is driving the analog-to-digital con-

verter (ADC). Today’s converters sample data at tens to

–INB RG

+OUTFILTERED hundreds of Megahertz with up to 16 bits of resolution. With 25Ω

each sample cycle, the switching of the internal ADC sample

and hold injects charge into the output of the driver which

must absorb the charge and settle its output before the next

sample is taken. This charge injection is inherent in nearly

25Ω all high speed, high resolution ADC topologies and must –OUTFILTERED

+INA RG

be considered when choosing a suitable driver.

RG The LT1993-x was designed specifically to drive high speed

ADCs to their full potential. With a 3.8n V/√Hz voltage noise

specification and –70d Bc of harmonic distortion at 70MHz ENABLE (2VP-P differential output), the LT1993-x meets and exceeds the requirements for driving high resolution high

DN366 F01

speed ADCs. Figure 2 shows an FFT of sampled data taken

VCC VEE

on a 70MHz input signal with the LT1993-2 driving an

1993-2: RF = 200Ω 1993-4: RF = 200Ω 1993-10: RF = 500Ω RG = 200Ω RG = 100Ω RG = 100Ω LTC®2249 sampling at 80Msps. BW = 800MHz BW = 900MHz BW = 700MHz

, LTC and LT are registered trademarks of Linear Technology Corporation. Figure 1. Block Diagram of the LT1993-x and the Differ- All other trademarks are the property of their respective owners.

ences Between the Gain Options. Input Impedance Is 200Ω for the 6d B Version and 100Ω for the Other Two Versions