DESIGN IDEAS L Dual ADC Driver with Tightly Matched Gain and Phase Raises Quadrature by Kris Lokere Demodulation Performance Introduction High speed differential amplifiers help to drive high performance analog-todigital converters (ADCs) by providing balanced drive to the differential input stage, setting the common mode level, absorbing the sampling-energy, and taking gain to reduce the output swing requirement of preceding circuitry. In applications involving multiple ADCs, a compact, tightly matched dual differential amplifier such as the LTC6420-20 can further simplify the design by reducing the uncertainty of channel-to-channel variations. The LTC6420-20 features two 1.8GHz differential amplifiers, each capable of driving 12-, 14- and 16bit pipeline ADCs at low noise and low distortion levels, even for input frequencies as high as 300MHz. The internal op amps are manufactured on a Silicon Germanium (SiGe) process and feature an ultra-low 1nV/√Hz noise density. Even after factoring in In applications involving multiple ADCs, a compact, tightly matched dual differential amplifier simplifies design by reducing the uncertainty of channelto-channel variations. amplifiers. For further space-savings, all gain and feedback resistors are included on-chip, setting a fixed voltage gain of 20dB. The LTC6420-20 consumes 80mA per channel from a supply as low as 2.85V. A 40mA version, the LTC6421-20, is available to save power at the expense of usable input bandwidth. Wideband Interface Between an I/Q Demodulator and the internal feedback resistors, the a Dual ADC total input noise density equals only 2.2nV/√Hz. The channel-to-channel gain matching is production tested and guaranteed to be better than ±0.25dB. Moreover, the monolithic design ensures that both phase and group delay remain tighly matched over the entire bandwidth of the amplifiers. The dual amplifier is available in a 3mm × 4mm QFN package, which saves space compared to two single When an IF or RF signal is applied to a demodulator such as the LT5575, the signal’s information is separated out into two baseband signals: “inphase” (I) and “quadrature” (Q). Both baseband signals need to be digitized simultaneously, and it is critical that the relationship of gain and phase between the two is maintained. Furthermore, if you want to maximize the dynamic range by driving the dual continued on page 35 3V 1000pF V+ A LTC6421-20 5V LT5575 75Ω 75Ω 27pF RF IN 2.5GHz 12pF 0.1µF 75Ω –INA 100Ω 0.1µF 12pF 0.1µF 1000Ω ENABLEA 12.5Ω +OUTA 40.2Ω 12.5Ω –OUTA 40.2Ω 10Ω VCM 56pF 1000Ω +INB 100Ω 1000Ω 10Ω 56pF V– 12.5Ω –OUTB 40.2Ω 12.5Ω +OUTB 40.2Ω 10Ω 56pF –INB 100Ω V+ B VOCMB 10Ω 56pF 1000Ω 27pF LO IN 2.5GHz VOCMA +INA 100Ω V– 27pF 75Ω 27pF 0.1µF 0.1µF 0.1µF ENABLEB VCM LTC2285 1000pF 0.1µF 3V Figure 1. The voltage gain of the LTC6421-20 reduces the output swing requirements of the LT5575 demodulator. This improves overall system linearity. Linear Technology Magazine • June 2008 33 DESIGN IDEAS L internal Schottky diode between CAP and BOOST, which saves an external component for applications with two or more series LEDs (see Figure 2 for a single LED solution). VIN 8V TO 32V VIN LT3592 ON SHDN BRIGHT OUT 4.7µF 5V 31.6k LTC6420, continued from page 33 ADC close to full scale, without driving the demodulator so hard that it causes excessive distortion, you need to insert some gain between the demodulator output and the ADC input. In Figure 1, the LTC6421-20 provides this gain, while the tight matching between its two channels contributes 48.7k 2.2MHz RT GND Conclusion a negligible amount of gain or phase error. The bandwidth and linearity of the LTC6421-20 ensures that 14-bit linearity (distortion less than –84dBc) is maintained to 50MHz and beyond, an important design criteria in digital-predistortion (DPD) circuits or wideband receivers. Paralleling Two Drivers to Lower the Noise Floor C4 0.1µF R5 49.9Ω 1/2 LTC6420-20 VIN R6 49.9Ω R7 49.9Ω 1/2 LTC6420-20 R8 49.9Ω VOCM R3 C2 12pF 10Ω C5 R4 12pF 10Ω The LT3592 makes 2-state bright/dim LED control simple and rugged. It is an ideal solution for applications such as automotive brake lights and flashing warning lights in industrial systems. Accurate control of the current levels makes LED brightness consistent across units in a given application regardless of varying LED forward voltage characteristics. Switching between the two current levels can be accomplished with either very low or very high voltage level digital signals. L In applications with only one ADC, you can hook-up the two channels of the dual amplifier in parallel, as shown in Figure 2. The main benefit of doing so is a reduction in noise, because the random noise contributions of each channel get averaged out. For example, input noise density (with inputs shorted) drops from 2.2nV/√Hz to 1.5nV/√Hz, a 3dB improvement in SNR if the driver were the dominant noise source. Conclusion LTC2208 C3 12pF –3dB FILTER BANDWIDTH = 120MHz R2 1k Figure 2. Connecting the two channels of the LTC6420-20 in parallel reduces the noise floor Linear Technology Magazine • June 2008 10k stand up to 36V, so they can be tied to the input voltage. Nevertheless, both pins have low voltage thresholds that allow them to be directly interfaced to low voltage microcontrollers. The LT3592 is not only useful for LED applications. It has a fully functional voltage control loop, and the current loop can be used as an accurate current clamp for voltage output applications. The voltage loop is also useful as a voltage clamp in case of an open LED fault. The transition between voltage and current control is stable and seamless. 3.3V C1 0.1µF VFB Figure 5. A 5V power supply with a 500mA current limit 3.3V R1 1.21k MBRA140 DA CAP 0.4Ω In addition to an internal switch current limit circuit, the LT3592 includes a catch diode current sense limit function that protects the circuit during start-up at high input voltages. Simply connect the anode of the Schottky catch diode to the DA pin, and the LT3592 automatically reduces the oscillator frequency when the catch diode current is higher than 1A. The lower operating frequency prevents the inductor current from ramping up in an uncontrolled fashion and allows the switch current limit to be effective by avoiding minimum on time restrictions. The LT3592 also automatically reduces its operating frequency if the LED string shorts out, minimizing power dissipation in the part. The SHDN and BRIGHT pins are as rugged as the VIN pin and can with- 3.3V 0.1µF 4.7µH SW Rugged Solution for Tough Environments + – BOOST 1µF The LTC6420 features two high speed differential amplifiers in a small 3mm × 4mm QFN package, with guaranteed tight matching specs between the two channels. It is ideal for driving high frequency signals into dual ADCs, especially when board space is limited or when the magnitude and phase relationship between the signals must be preserved. L 35