EDNE August 2012 - (Page 18)

UNDERstaND thE Digital-OUtPUt OPtiONs fOR high-sPEED aDCs LArGe SySTemS Are USinG more And hiGher-Speed chAnneLS in Their AnALoG fronT endS, for which deSiGnerS mUST chooSe AmonG cmoS, LVdS, And cmL oUTpUTS. UnderSTAnd The key chArAcTeriSTicS, performAnce TrAde-offS, And LAyoUT reqUiremenTS of TheSe oUTpUTS. W imagE: iSTOCKPHOTO.COm By J on at ha n h arris • analog Devices inc ith a multitude of ADC choices available for designers, they must consider what type of digital-data outputs to use: CMOS (complementary metal-oxide semiconductor), LVDS (low-voltage differential signaling), or CML (current-mode logic). Each of the digital-output types used in ADCs has advantages and disadvantages that designers should consider for their applications. These factors depend on the sampling rate and the resolution of the ADC, the output data rates, and the power requirements of the system design, among other factors. CMOS driverS CMOS digital outputs are common in ADCs with sample rates lower than 200M samples/sec. A typical CMOS driver comprises one NMOS transistor and one PMOS transistor, which connect between the drain-to-drain, or power-supply, voltage, VDD, and ground (Figure 1a). This structure results in an inversion in the output. Alternatively, you can use a back-to-back structure to avoid the inversion in the output (Figure 1b). The CMOS output driver has a high-impedance input and a low-impedance output. At the input to the driver, the impedance of the gates of the two CMOS transistors is quite high because the gate oxide isolates the gate from any conducting material. The impedances at the input can range from kilohms to megohms. At the driver’s output, the drain current, ID, which is typically small, governs the impedance. In this case, the impedance is usually less than a few hundred ohms. The voltage levels for CMOS swing from approximately the power-supply voltage to ground and can therefore be large, depending on the power-supply voltage. Because the input impedance is high and the output impedance is relatively low, one CMOS output can typically drive multiple CMOS inputs. CMOS outputs also have low static current. Significant current flow occurs only during a switching event on the CMOS driver. When the driver is in either a low state—that is, pulled to ground—or a high state—that is, pulled to the power-supply voltage—little current flows through the driver. However, when the driver is switching from a low state to a high state or from a high state to a low state, a momentary low-resistance path occurs from the power-supply voltage to ground. This transient current 18 EDN EUROPE | AUGUST 2012 www.edn-europe.com http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE August 2012

Messe München
Messe München
Cover
Contents
Farnell
Microchip
Digi-key
Masthead
Comment
Pulse
Digi-key
Test & Measurement
Agilent Technologies
Understand the digital-output options for high-speed ADCs
Digi-key
Baker's best
Omicron
Cover Story
Digi-key
Generating spatial audio from portable products
Supply Chain
Design Idea
Product Roundup
Tales From The Cube

EDNE August 2012