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LTC1051 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1051 Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page 7 LTC1051/LTC1053 10513fa As the ambient temperature rises, the leakage current of the input protection devices increases, while the charge injection component of the bias current, for all practical purposes, stays constant. At elevated temperatures (above 85°C) the leakage current dominates and the bias current of both inputs assumes the same sign. The charge injection at the op amp input pins will cause small output spikes. This phenomenon is often referred to as “clock feedthrough” and can be easily observed when the closed-loop gain exceeds 10V/V (Figure 2). The mag- nitude of the clock feedthrough is temperature indepen- dent but it increases when the closed-loop gain goes up, when the source resistance increases and when the gain setting resistors increase (Figure 2a, 2b). It is important to note that the output small spikes are centered at 0V level and do not add to the output offset error budget. For instance, with RS = 1MΩ, the typical output offset voltage of Figure 2c is: VOS(OUT) ≈ 108 • IB+ + 101VOS(IN) A 10pA bias current will yield an output of 1mV ±100µV. The output clock feedthrough can be attenuated by lower- ing the value of the gain setting resistors, i.e. R2 = 10k, R1 = 100Ω, instead of 100k and 1k (Figure 2). Clock feedthrough can also be attenuated by adding a capacitor across the feedback resistor to limit the circuit bandwidth below the internal sampling frequency (Figure 3). Input Capacitance The input capacitance of the LTC1051/LTC1053 op amps is approximately 12pF. When the LTC1051/LTC1053 op amps are used with feedback factors approaching unity, the feedback resistor value should not exceed 7k for industrial temperature range and 5k for military tempera- ture range. If a higher feedback resistor value is required, a feedback capacitor of 20pF should be placed across the feedback resistor. Note that the most common circuits with feedback factors approaching unity are unity gain followers and instrumentation amplifier front ends. (See Figure 4.) Figure 2. Clock Feedthrough Figure 3. Adding a Feedback Capacitor to Eliminate Clock Feedthrough – + 1/2 LTC1051 RS 1051/53 F02 (c) 100µs/DIV (b) 100µs/DIV (a) R1 1k R2 100k RS = 0, AV =11V/V 20mV/DIV RS = 0, AV =101V/V 20mV/DIV RS = 100k, AV =11V/V 20mV/DIV RS = 100k, AV =101V/V 20mV/DIV Figure 4. Operating the LTC1051 with Feedback Factors Approaching Unity – + 1/2 LTC1051 1051/53 F04 R1 R2 < 7k, IF R1 > >R2 1 2 3 – + 1/2 LTC1051 RS 1051/53 F03 R1 1k R2 100k C 1000pF 1 2 3 100µs/DIV RS = 100k AV =101V/V RS = 1MΩ AV =101V/V APPLICATIO S I FOR ATIO |
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