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LTC2202 Datasheet(PDF) 21 Page - Linear Technology |
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LTC2202 Datasheet(HTML) 21 Page - Linear Technology |
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21 / 32 page  LTC2203/LTC2202 21 22032fb Figure 5 demonstrates the use of an LTC1994 differential amplifier to convert a single ended input signal into a differential input signal. The advantage of this method is that it provides low frequency input response; however, the limited gain bandwidth of any op amp will limit the SFDR at high input frequencies. The 25Ω resistors and 12pF capacitor on the analog inputs serve two purposes: isolating the drive circuitry from the sample-and-hold charging glitches and limiting the wideband noise at the converter input. Reference Operation Figure 6 shows the LTC2203/LTC2202 reference circuitry consisting of a 2.5V bandgap reference, a programmable gain amplifier and control circuit. The LTC2203/LTC2202 has three modes of reference operation: Internal Refer- ence, 1.25V external reference or 2.5V external reference. To use the internal reference, tie the SENSE pin to VDD. To use the external reference, simply apply either a 1.25V or 2.5V reference voltage to the SENSE input pin. Both 1.25V and 2.5V applied to SENSE will result in a full scale range of 2.5VP-P (PGA = 0). A 1.25V output, VCM, is provided for a common mode bias for input drive circuitry. An external bypass capacitor is required for the VCM output. This provides a high frequency low impedance path to ground for internal and external circuitry. This is also the compensation capacitor for the reference; it will not be stable without this capacitor. The minimum value required for stability is 2.2µF. APPLICATIO S I FOR ATIO The internal programmable gain amplifier provides the internal reference voltage for the ADC. This amplifier has very stringent settling requirements and is not accessible for external use. Figure 5. DC Coupled Input with Differential Amplifier VCM LTC2203/02 22032 F05 – – + + CM LT1994 499 Ω 100pF 100pF 100pF 2.2 μF 499 Ω 523 Ω 499 Ω 53.6 Ω AIN + AIN – Figure 6. Reference Circuit PGA 1.25V SENSE VCM BUFFER INTERNAL ADC REFERENCE RANGE SELECT AND GAIN CONTROL 2.5V BANDGAP REFERENCE 2.2 μF TIE TO VDD TO USE INTERNAL 2.5V REFERENCE OR INPUT FOR EXTERNAL 2.5V REFERENCE OR INPUT FOR EXTERNAL 1.25V REFERENCE 22032 F07 LTC2203/02 Figure 7. A 2.25V Range ADC with an External 2.5V Reference VCM SENSE 1.25V 3.3V 2.2 μF 2.2 μF 1 μF 22032 F08 LTC2203/02 LT1461-2.5 2 6 4 The SENSE pin can be driven ±5% around the nominal 2.5V or 1.25V external reference input. This adjustment range can be used to trim the ADC gain error or other system gain errors. When selecting the internal reference, the SENSE pin should be tied to VDD as close to the converter as possible. If the sense pin is driven externally it should be bypassed to ground as close to the device as possible with at least a 1µF ceramic capacitor. S |
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Similar Description - LTC2202 |
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