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LTC1250CN8 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1250CN8 Datasheet(HTML) 6 Page - Linear Technology |
6 / 8 page 6 LTC1250 where AV = closed-loop gain. Note that CF is not dependent on the value of RF. Circuits with higher gain (AV > 50) or low loop impedance should not require CF for stability. frequency, approaching the best bipolar op amps at 10Hz and surpassing them below 1Hz (Figure 1). All this is accomplished in an industry-standard pinout; the LTC1250 requires no external capacitors, no nulling or clock sig- nals, and conforms to industry-standard 8-pin DIP and 8- pin SOIC packages. Input Capacitance and Compensation The large input transistors create a parasitic 55pF capaci- tance from each input to V+. This input capacitance will react with the external feedback resistors to form a pole which can affect amplifier stability. In low gain, high impedance configurations, the pole can land below the unity-gain frequency of the feedback network and degrade phase margin, causing ringing, oscillation, and other unpleasantness. This is true of any op amp, however, the 55pF capacitance at the LTC1250’s inputs can affect stability with a feedback network impedance as low as 1.9k. This effect can be eliminated by adding a capacitor across the feedback resistor, adding a zero which cancels the input pole (Figure 2). The value of this capacitor should be: S APPLICATI I FOR ATIO fully cancel the 1/f noise spectrum and the low frequency noise of the part will rise. If the loop is underdamped (large RF, no CF) it will ring for more than 150µs and the noise and offset will suffer. The solution is to add CF as above but beware! Too large a value of CF will overdamp the loop, again preventing it from reaching a final value by the 150 µs deadline. This condition doesn’t affect the LTC1250’s offset or output stability, but 1/f noise begins to rise. As a rule of thumb, the RFCF feedback pole should be ≥ 7kHz (1/150µs, the frequency at which the loop settles) for best 1/f perfor- mance; values between 100pF and 500pF work well with feedback resistors below 100k. This ensures adequate gain at 7kHz for the LTC1250 to properly null. High value feedback resistors (above 1M) may require experimenta- tion to find the correct value because parasitics, both in the LTC1250 and on the PC board, play an increasing role. Low value resistors (below 5k) may not require a capaci- tor at all. Input Bias Current The inputs of the LTC1250, like all zero-drift op amps, draw only small switching spikes of AC bias current; DC leakage current is negligible except at very high tempera- tures. The large front-end transistors cause switching spikes 3 to 4 times greater than standard zero-drift op amps: the ±50pA bias current spec is still many times better than most bipolar parts. The spikes don’t match from one input pin to the other, and are sometimes (but not always) of opposite polarity. As a result, matching the impedances at the inputs (Figure 3) will not cancel the bias current, and may cause additional errors. Don’t do it. – + CF RIN 1250 F02 LTC1250 RF CP Figure 2. CF Cancels Phase Shift Due to Parasitic CP Larger values of CF, commonly used in band-limited DC circuits, may actually increase low frequency noise. The nulling circuitry in the LTC1250 closes a loop that includes the external feedback network during part of its cycle. This loop must settle to its final value within 150 µs or it will not C pF A F V ≥ 55 – + RIN 1250 F03 LTC1250 RF Figure 3. Extra Resistor Will Not Cancel Bias Current Errors |
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