Datasheet LTC1250 (Analog Devices) - 6

LTC1250
O U U W U APPLICATI S I FOR ATIO Input Noise
stability with a feedback network impedance as low as 1.9k. This effect can be eliminated by adding a capacitor The LTC1250, like all CMOS amplifiers, exhibits two types across the feedback resistor, adding a zero which cancels of low frequency noise: thermal noise and 1/f noise. The the input pole (Figure 2). The value of this capacitor should LTC1250 uses several design modifications to minimize be: these noise sources. Thermal noise is minimized by rais- ing the gM of the front-end transistors by running them at pF 55 high bias levels and using large transistor geometries. 1/f CF ≥ A noise is combated by optimizing the zero-drift nulling loop V to run at twice the 1/f corner frequency, allowing it to where AV = closed-loop gain. Note that CF is not dependent reduce the inherently high CMOS 1/f noise to near thermal on the value of RF. Circuits with higher gain (AV > 50) or levels at low frequencies. The resultant noise spectrum is low loop impedance should not require CF for stability. quite low at frequencies below the internal 5kHz clock frequency, approaching the best bipolar op amps at 10Hz CF and surpassing them below 1Hz (Figure 1). All this is accomplished in an industry-standard pinout; the LTC1250 RF requires no external capacitors, no nulling or clock sig- RIN nals, and conforms to industry-standard 8-pin DIP and – 8-pin SO packages. LTC1250 CP + 1250 F02 80 VS = ±5V 70 RS = 10Ω
Figure 2. CF Cancels Phase Shift Due to Parasitic CP
OP-27 OP-07 Hz) 60 √ 50 Larger values of CF, commonly used in band-limited DC LTC1250 40 circuits, may actually increase low frequency noise. The nulling circuitry in the LTC1250 closes a loop that includes 30 the external feedback network during part of its cycle. This 20 VOLTAGE NOISE (nV/ loop must settle to its final value within 150µs or it will not 10 fully cancel the 1/f noise spectrum and the low frequency 0 noise of the part will rise. If the loop is underdamped (large 0.01 0.1 1 FREQUENCY (Hz) RF, no CF) it will ring for more than 150µs and the noise and LTC1250 F01 offset will suffer.
Figure 1. Voltage Noise vs Frequency
The solution is to add CF as above but beware! Too large
Input Capacitance and Compensation
a value of CF will overdamp the loop, again preventing it from reaching a final value by the 150µs deadline. This The large input transistors create a parasitic 55pF capaci- condition doesn’t affect the LTC1250’s offset or output tance from each input to V+. This input capacitance will stability, but 1/f noise begins to rise. As a rule of thumb, react with the external feedback resistors to form a pole the R which can affect amplifier stability. In low gain, high FCF feedback pole should be ≥ 7kHz (1/150µs, the frequency at which the loop settles) for best 1/f perfor- impedance configurations, the pole can land below the mance; values between 100pF and 500pF work well with unity-gain frequency of the feedback network and degrade feedback resistors below 100k. This ensures adequate phase margin, causing ringing, oscillation, and other gain at 7kHz for the LTC1250 to properly null. High value unpleasantness. This is true of any op amp, however, the feedback resistors (above 1M) may require experimenta- 55pF capacitance at the LTC1250’s inputs can affect tion to find the correct value because parasitics, both in the 1250fb 6