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LTC2356CMSE-12 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2356CMSE-12 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page 11 LTC2356-12/LTC2356-14 2356f INPUT FILTERING AND SOURCE IMPEDANCE The noise and the distortion of the input amplifier and other circuitry must be considered since they will add to the LTC2356-12/LTC2356-14 noise and distortion. The small-signal bandwidth of the sample-and-hold circuit is 50MHz. Any noise or distortion products that are present at the analog inputs will be summed over this entire bandwidth. Noisy input circuitry should be filtered prior to the analog inputs to minimize noise. A simple 1-pole RC filter is sufficient for many applications. For example, Figure 1 shows a 47pF capacitor from AIN+ to ground and a 51 Ω source resistor to limit the input bandwidth to 47MHz. The 47pF capacitor also acts as a charge reservoir for the input sample-and-hold and isolates the ADC input from sampling-glitch sensitive circuitry. High quality ca- pacitors and resistors should be used since these compo- nents can add distortion. NPO and silvermica type dielectric capacitors have excellent linearity. Carbon surface mount resistors can generate distortion from self heating and from damage that may occur during soldering. Metal film surface mount resistors are much less susceptible to both problems. When high amplitude unwanted signals are close in frequency to the desired signal frequency, a multiple pole filter is required. High external source resis- tance, combined with the 13pF of input capacitance, will reduce the rated 50MHz bandwidth and increase acquisi- tion time beyond 39ns. Figure 1. RC Input Filter inverting input. The ±1.25V range is also ideally suited for AC-coupled signals in single supply applications. Figure 2 shows how to AC couple signals in a single supply system without needing a mid-supply 1.5V external reference. The DC common mode level is supplied by the previous stage that is already bounded by the single supply voltage of the system. The common mode range of the inputs extend from ground to the supply voltage VDD. If the difference between the AIN+ and AIN– inputs exceeds 1.25V, the output code will stay fixed at zero and all ones and if this difference goes below –1.25V, the output code will stay fixed at one and all zeros. APPLICATIO S I FOR ATIO 10 µF VCM 1.5V DC 11 3 AIN – LTC2356-12/ LTC2356-14 AIN + 47pF 2 1 51 Ω GND VREF 2356 F01 + AIN + C4 10 µF 2356 F02 R2 1.6k C2 1 µF C1 1 µF C1, C2: FILM TYPE C3: COG TYPE C4: CERAMIC BYPASS R1 1.6k 1 2 3 LTC2356-12/ LTC2356-14 AIN – VREF R3 51 Ω C3 56pF VIN Figure 2. AC Coupling of AC Signals with 1kHz Low Cutoff Frequency INPUT RANGE The analog inputs of the LTC2356-12/LTC2356-14 may be driven fully differentially with a single supply. Each input may swing up to 2.5VP-P individually. When using the internal reference, the non-inverting input should never be more than 1.25V more positive or more negative than the INTERNAL REFERENCE The LTC2356-12/LTC2356-14 has an on-chip, tempera- ture compensated, bandgap reference that is factory trimmed to 2.5V to obtain a bipolar ±1.25V input span. The reference amplifier output VREF, (Pin 3) must be bypassed with a capacitor to ground. The reference amplifier is stable with capacitors of 1 µF or greater. For the best noise performance, a 10 µFceramicora10µFtantaluminparallel with a 0.1 µFceramicisrecommended.TheVREFpincanbe overdriven with an external reference as shown in Figure 3. Overdriving VREF Pin with an External Reference GND LTC2356-12/ LTC2356-14 LT1790-3 VREF 10 µF 3.5V TO 18V 11 3 3V 2356 F03 |
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