Datasheet LTC1250 (Analog Devices) - 7

LTC1250
O U U W U APPLICATI S I FOR ATIO
LTC1250 and on the PC board, play an increasing role. when the LTC1250’s output is heavily loaded, the chip Low value resistors (below 5k) may not require a capaci- may dissipate substantial power, raising the temperature tor at all. of the package and aggravating thermocouples at the inputs. Although the LTC1250 will maintain its specified
Input Bias Current
accuracy under these conditions, care must be taken in the layout to prevent or compensate circuit errors. Be The inputs of the LTC1250, like all zero-drift op amps, especially careful of air currents when measuring low draw only small switching spikes of AC bias current; DC frequency noise; nearby moving objects (like people) can leakage current is negligible except at very high tempera- create very large noise peaks with an unshielded circuit tures. The large front-end transistors cause switching board. For more detailed explanations and advice on how spikes 3 to 4 times greater than standard zero-drift op to avoid these errors, see the LTC1051/LTC1053 data amps: the ±50pA bias current spec is still many times sheet. better than most bipolar parts. The spikes don’t match from one input pin to the other, and are sometimes (but
Sampling Behavior
not always) of opposite polarity. As a result, matching the impedances at the inputs (Figure 3) will not cancel the The LTC1250’s zero-drift nulling loop samples the input at bias current, and may cause additional errors. Don’t do it. ≈ 5kHz, allowing it to process signals below 2kHz with no aliasing. Signals above this frequency may show aliasing RF behavior, although wideband internal circuitry generally RIN keeps errors to a minimum. The output of the LTC1250 – will have small spikes at the clock frequency and its LTC1250 harmonics; these will vary in amplitude with different + 1250 F03 feedback configurations. Low frequency or band-limited systems should not be affected, but systems with higher bandwidth (oversampling A/Ds, for example) may need to filter out these clock artifacts. Output spikes can be
Figure 3. Extra Resistor Will Not Cancel Bias Current Errors
minimized with a large feedback capacitor, but this will
Output Drive
adversely affect noise performance (see Input Capaci- tance and Compensation on the previous page). Applica- The LTC1250 includes an enhanced output stage which tions which require spike-free output in addition to mini- provides nearly symmetrical output source/sink currents. mum noise will need a low-pass filter after the LTC1250; This output is capable of swinging a minimum of ±4V into a simple RC will usually do the job (Figure 4). The a 1k load with ±5V supplies, and can sink or source >20mA LTC1051/LTC1053 data sheet includes more information into low impedance loads. Lightly loaded (RL ≥100k), the about zero-drift amplifier sampling behavior. LTC1250 will swing to within millivolts of either rail. In C single supply applications, it will typically swing 4.3V into F a 1k load with a 5V supply. RF
Minimizing External Errors
– The input noise, offset voltage, and bias current specs for 47k LTC1250 the LTC1250 are all well below the levels of circuit board + 0.01 parasitics. Thermocouples between the copper pins of the LTC1250 and the tin/lead solder used to connect them can 1250 F04 overwhelm the offset voltage of the LTC1250, especially
Figure 4. RC Output Pole Limits Bandwidth to 330Hz
if a soldering iron has been around recently. Note also that 1250fb 7