LTC6420 - Dual Matched 1.8GHz Differential Amplifiers / ADC Drivers
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Introduction of the features of the LTC6420, a dual matched, high-speed differential amplifier, and its typical applications
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LTC6420 - Dual Matched 1.8GHz Differential Amplifiers / ADC Drivers
- 10. Test Circuit 1
- 11. Test Circuit 2
- 12. Driving A/D Converters Figure1 - Dual ADC Driver for Wideband Direct-Conversion Receivers Figure2 - Parallel ADC Drivers to reduce wideband noise
Editor's Notes
- Welcome to the training module on Linear Technology LTC6420 - Dual Matched 1.8GHz Differential Amplifiers / ADC Drivers. This training module introduces the features of the LTC6420, a dual matched, high-speed differential amplifier, and its typical applications.
- The LTC6420-20 is a dual high-speed differential amplifier targeted at processing signals from DC to 300MHz. The part has been specifically designed to drive 12-, 14- and 16-bit ADCs with low noise and low distortion, but can also be used as a general-purpose broadband gain block. It is a high-speed dual differential ADC drivers with guaranteed gain matching, and exceptional noise and distortion performance. With guaranteed matched gain of ±0.25dB and matched phase of ±0.1° (typ), the LTC6420-20 reduces errors in multichannel systems such as IQ demodulation or diversity receivers. Channel separation is 80dB at 100MHz.
- This slide shows about the absolute rating of the device LTC6420: typical power supply of 3.6V, 10 mA current and temperature ratings.
- This device can be used to drive differential ADC, single ended to differential conversion, intermediate frequency sampling receivers.
- Each of the two channels of the LTC6420-20 is composed of a fully differential amplifier with on chip feedback and output common mode voltage control circuitry. Differential gain and input impedance are set by 100Ω/1000Ω (R G & R F ) resistors in the feedback network. Small output resistors of 12.5Ω improve the circuit stability over various load conditions. The LTC6420-20 provides an output common mode voltage set by VOCM, which allows driving an ADC directly without external components such as a transformer or AC coupling capacitors.
- This page shows information about the pin description of LTC6420 device. It is available in 20 pin QFN package. The pins that are actively used are: +INA, –INA, –INB, +INB, V+ A , V+ B, V– , ENABLEA, ENABLEB, –OUTA, +OUTA, –OUTB, +OUTB, VOCMA, VOCMB, Exposed Pad.
- The differential input impedance of the LTC6420-20 is 200Ω. If a 200Ω source impedance is unavailable, then the differential inputs may need to be terminated to a lower value impedance, e.g. 50Ω, in order to provide an impedance match for the source. Several choices are available. One approach is to use a differential shunt resistor (Figure 1). Another approach is to employ a wide band transformer (Figure 2). Both methods provide a wide band impedance match. The termination resistor or the transformer must be placed close to the input pins in order to minimize the reflection due to input mismatch. Alternatively, one could apply a narrowband impedance match at the inputs of the LTC6420-20 for frequency selection and/or noise reduction.
- The LTC6420-20 can drive an ADC directly without external output impedance matching. Alternatively, the differential output impedance of 25Ω can be matched to a higher value impedance. The output common mode voltage is set by the V OCM pin, which is a high impedance input. It can track V OCM in a range from 1.1V to 1.6V. The output common mode voltage is capable of tracking VOCM in a range from 1.1V to 1.6V. The bandwidth of VOCM control is typically 15MHz, which is dominated by a low pass filter connected to the VOCM pin and is aimed to reduce common mode noise generation at the outputs.
- The circuit is a two-port demonstration circuit for the LTC6420 family. This circuit includes input and output transformers (baluns) for single-ended-to-differential conversion and impedance transformation, allowing direct hook-up to a 2-port network analyzer. There are also series resistors at the output to avoid loading the amplifier directly with a 50Ω load. Due to the input and output transformers, the –3dB bandwidth is reduced from 1.8GHz to approximately 1.3GHz.
- This circuit uses a 4-port network analyzer to measure S-parameters and gain/phase response. This removes the effects of the wideband baluns and associated circuitry, for a true picture of the >1GHz S-parameters and AC characteristics.
- The LTC6420-20 has been specifically designed to interface directly with high speed A/D converters. The figure 1 shows the LTC6420-20 driving an LTC2285, which is a dual 14-bit, 125Msps ADC. The V OCM pins of the LTC6420-20 are connected to the V CM pins of the LTC2285, which provide a DC voltage level of 1.5V. Both ICs are powered from the same 3V supply voltage. The outputs of the LTC6420-20 can be connected directly to the analog inputs of an ADC, or a simple lowpass or bandpass filter network may be inserted to reduce out-of-band noise. In applications with only one ADC, the user 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.
- Thank you for taking the time to view this presentation on “ LTC6420 - Dual Matched 1.8GHz Differential Amplifiers / ADC Drivers” . If you would like to learn more or go on to purchase some of these devices, you may either click on the part list link, or simple call our sales hotline. For more technical information you may either visit the Linear Technology site, or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.