Datasheet LTC1052, LTC7652 (Analog Devices) - 7

LTC1052/LTC7652
U THEORY OF OPERATIO
power supply are also nulled. For nulling to take place, the For frequencies above this pole, I2 is: offset voltage, common mode voltage and power supply I 1 • SC1 must not change at a frequency which is high compared to 2 = VIN gm6 • SC2 the frequency response of the nulling loop. and C1 I
AC OPERATION AND ALIASING ERRORS
1 – I2 = VIN gm1 – VIN gm6 • C2 The LTC1052 is very carefully designed so that g So far, the DC performance of the LTC1052 has been m1 = gm6 and C1 = C2. Substituting these values in the above equa- explained. As the input signal frequency increases, the tion shows I problem of aliasing must be addressed. Aliasing is the 1 – I2 = 0. spurious formation of low and high frequency signals The gm6 input stage, with Cl and C2, not only filters the caused by the mixing of the input signal with the sampling input to the sampling loop, but also acts as a high frequency, f frequency path to give the LTC1052 good high frequency S. The frequency of the error signals, fE, is: response. The unity-gain cross frequencies for both the fE = fS ±fI DC path and high frequency path are identical where fI = input signal frequency. 1 [f3dB = (g (g 2 m1/C1) = 1 2 m6/C2)] Normally it is the difference frequency (f π π S – fI ) which is of thereby making the frequency response smooth and con- concern because the high frequency (fS + fI) can be easily tinuous while eliminating sampling noise in the output as filtered. As the input frequency approaches the sampling the loop transitions from the high gain DC loop to the high frequency, the difference frequency approaches zero and frequency loop. will cause DC errors—the exact problem that the zero-drift amplifier is meant to eliminate. The typical curves show just how well the amplifier works. The output spectrum shows that the difference frequency The solution is simple; filter the input so the sampling loop (f never sees any frequency near the sampling frequency. I–fS = 100Hz) is down by 80dB and the frequency response curve shows no abnormalities or perturbations. At a frequency well below the sampling frequency, the Also note the well-behaved small and large-signal step LTC1052 forces I1 to equal I2 (see Figure 1b). This makes responses and the absence of the sampling frequency in δ l zero, thus the gain of the sampling loop zero at this and the output spectrum. If the dynamics of the amplifier higher frequencies (i.e., a low pass filter). The corner (i.e., slew rate and overshoot), depend on the sampling frequency of this low pass filter is set by the output stage clock, the sampling frequency will appear in the output pole (1/RL4 gm5 RL5 C2). spectrum. C1 S3 VREF C2 + IN S1 + S2 – g – m1 + gm2 gm4 + gm5 V – + OUT – IN – RL1 RL2 CEXT B RL4 RL5 C V EXT A NULL gm3 – V – gm6 LTC1052/7652 • TPC13 +
Figure 1a. LTC1052 Block Diagram Auto Zero Cycle
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