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LTC2355-14 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2355-14 Datasheet(HTML) 11 Page - Linear Technology |
11 / 18 page LTC2355-12/LTC2355-14 2355fa applications inFormation 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 LTC2355-12/LTC2355-14 noise and distortion. The small-signal bandwidth of the sample-and-hold circuit is 50MHz. Any noise or distortion products that are pres- ent 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+togroundanda 51Ωsourceresistortolimittheinputbandwidthto47MHz. The 47pF capacitor also acts as a charge reservoir for the input sample-and-hold and isolates the ADC input from sampling-glitchsensitivecircuitry.Highqualitycapacitors and resistors should be used since these components 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.Whenhighamplitudeunwantedsignalsareclose in frequency to the desired signal frequency, a multiple pole filter is required. High external source resistance, combined with the 13pF of input capacitance, will reduce the rated 50MHz bandwidth and increase acquisition time beyond 39ns. INPUT RANGE The analog inputs of the LTC2355-12/LTC2355-14 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 noninverting input should never be more than 2.5V more positive than the inverting input. The 0V to 2.5V range is also ideally suited for single-ended input use with single supply applications. The common mode range of the inputs extend from ground to the sup- ply voltage VDD. If the difference between the AIN+ and AIN– inputs exceeds 2.5V, the output code will stay fixed at all ones and if this difference goes below 0V, the ouput code will stay fixed at all zeros. INTERNAL REFERENCE TheLTC2355-12/LTC2355-14hasanon-chip,temperature compensated, bandgap reference that is factory trimmed to 2.5V to obtain a unipolar 0V to 2.5V 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 1µF or greater. For the best noise performance,a10µFceramicora10µFtantaluminparallel with a 0.1µF ceramic is recommended. The VREFpincanbe overdriven with an external reference as shown in Figure 2. The voltage of the external reference must be higher than the 2.5V output of the internal reference. The recom- mended range for an external reference is 2.55V to VDD. An external reference at 2.55V will see a DC quiescent load of 0.75mA and as much as 3mA during conversion. Figure 1. RC Input Filter Figure 2. Overdriving VREF Pin with an External Reference 10µF 11 3 AIN– LTC2355-12/ LTC2355-14 AIN+ 47pF 2 1 51 GND VREF 2355 F01 GND LTC2355-12/ LTC2355-14 LT1790-3 VREF 10µF 3.5V TO 18V 11 3 3V 2355 F02 |
Similar Part No. - LTC2355-14_15 |
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Similar Description - LTC2355-14_15 |
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