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LTC2439-1 Datasheet(PDF) 26 Page - Linear Technology |
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LTC2439-1 Datasheet(HTML) 26 Page - Linear Technology |
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26 / 28 page  LTC2439-1 26 24391f First, a change in fEOSC will result in a proportional change in the internal notch position and in a reduction of the converter differential mode rejection at the power line frequency. In many applications, the subsequent perfor- mance degradation can be substantially reduced by rely- ing upon the LTC2439-1’s exceptional common mode rejection and by carefully eliminating common mode to differential mode conversion sources in the input circuit. The user should avoid single-ended input filters and should maintain a very high degree of matching and symmetry in the circuits driving the IN+ and IN– pins. Second, the increase in clock frequency will increase proportionally the amount of sampling charge transferred through the input and the reference pins. If large external input and/or reference capacitors (CIN, CREF) are used, the previous section provides formulae for evaluating the effect of the source resistance upon the converter perfor- mance for any value of fEOSC. If small external input and/ or reference capacitors (CIN, CREF) are used, the effect of the external source resistance upon the LTC2439-1 typical performance can be inferred from Figures 14, 15, 19 and 20 in which the horizontal axis is scaled by 139,800/fEOSC. Third, an increase in the frequency of the external oscilla- tor above 460800Hz (a more than 3 × increaseintheoutput data rate) will start to decrease the effectiveness of the internal autocalibration circuits. This will result in a pro- gressive degradation in the converter accuracy and linear- ity. Typical measured performance curves for output data rates up to 100 readings per second are shown in Fig- ures 24, 25, 26, 27, 28 and 29. In order to obtain the highest possible level of accuracy from this converter at output data rates above 20 readings per second, the user is advised to maximize the power supply voltage used and to limit the maximum ambient operating temperature. In certain circumstances, a reduction of the differential refer- ence voltage may be beneficial. Increasing Input Resolution by Reducing Reference Voltage The resolution of the LTC2439-1 can be increased by reducing the reference voltage. It is often necessary to amplify low level signals to increase the voltage resolution of ADCs that cannot operate with a low reference voltage. The LTC2439-1 can be used with reference voltages as low as 100mV, corresponding to a ±50mVinputrangewithfull 16-bit resolution. Reducing the reference voltage is func- tionally equivalent to amplifying the input signal, however no amplifier is required. The LTC2439-1 has a 76 µV LSB when used with a 5V reference, however the thermal noise of the inputs is 1 µVRMS and is independent of reference voltage. Thus reducing the reference voltage will increase the resolution at the inputs as long as the LSB voltage is significantly larger than 1 µVRMS. A 325mV reference corresponds to a 5 µV LSB, which is approximately the peak-to-peak value of the 1 µVRMSinputthermalnoise.Atthispoint,theoutput code will be stable to ±1LSB for a fixed input. As the reference is decreased further, the measured noise will approach 1 µVRMS. Figure 30 shows two methods of dividing down the reference voltage to the LTC2439-1. Where absolute accu- racy is required, a precision divider such as the Vishay MPM series dividers in a SOT-23 package may be used. A 51:1 divider provides a 98mV reference to the LTC2439-1 from a 5V source. The resulting ±49mV input range and 1.5 µV LSB is suitable for thermocouple and 10mV full- scale strain gauge measurements. If high initial accuracy is not critical, a standard 2% resistor array such as the Panasonic EXB series may be used. Single package resistor arrays provide better tem- perature stability than discrete resistors. An array of eight resistors can be configured as shown to provide a 294mV reference to the LTC2439-1 from a 5V source. The fully differential property of the LTC2439-1 reference terminals allow the reference voltage to be taken from four central resistors in the network connected in parallel, minimizing drift in the presence of thermal gradients. This is an ideal reference for medium accuracy sensors such as silicon micromachined pressure and force sensors. These de- vices typically have accuracies on the order of 2% and full- scale outputs of 50mV to 200mV. APPLICATIO S I FOR ATIO |
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Similar Description - LTC2439-1 |
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