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TC7652CPA Datasheet(PDF) 7 Page - Microchip Technology |
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TC7652CPA Datasheet(HTML) 7 Page - Microchip Technology |
7 / 16 page © 2002 Microchip Technology Inc. DS21464B-page 7 TC7652 4.3 Output Stage/Load Driving The output circuit is high impedance (about 18k Ω). With lesser loads, the chopper amplifier behaves somewhat like a transconductance amplifier with an open-loop gain proportional to load resistance. (For example, the open-loop gain is 17dB lower with a 1k Ω. load than with a 10k Ω load.) If the amp is used only for DC, the DC gain is typically greater than 120dB (even witha1k Ω load), and this lower gain is inconsequential. For wide band, the best frequency response occurs with a load resistor of at least 10k Ω. This produces a 6dB/octave response from 0.1Hz to 2MHz, with phase shifts of less than 2 degrees in the transition region, where the main amplifier takes over from the null ampli- fier. FIGURE 4-1: CONNECTION OF INPUT GUARDS 4.4 Thermoelectric Effects The thermoelectric (Seebeck) effects in thermocouple junctions of dissimilar metals, alloys, silicon, etc. limit ultra high precision DC amplifiers. Unless all junctions are at the same temperature, thermoelectric voltages around 0.1 µV/°C (up to tens of µV/°C for some materi- als) are generated. To realize the low offset voltages of the chopper, avoid temperature gradients. Enclose components to eliminate air movement, especially from power dissipating elements in the system. Where pos- sible, use low thermoelectric co-efficient connections. Keep power supply voltages and power dissipation to a minimum. Use high impedance loads and seek maxi- mum separation from surrounding heat disipating ele- ments. 4.5 Guarding To benefit from TC7652 low input currents, take care assembling printed circuit boards. Clean boards with alcohol or TCE and blow dry with compressed air. To prevent contamination, coat boards with epoxy or sili- cone rubber. Even if boards are cleaned and coated, leakage cur- rents may occur because input pins are next to pins at supply potentials. To reduce this leakage, use guarding to lower the voltage difference between the inputs and adjacent metal runs. The guard (a conductive ring sur- rounding inputs) is connected to a low impedance point at about the same voltage as inputs. The guard absorbs leakage currents from high voltage pins. The 14-pin dual-in-line arrangement simplifies guard- ing. Like the LM108 pin configuration (but unlike the 101A and 741), pins next to inputs are not used. Input Output R2 R1 Inverting Amplifier Input Output Follower Input Output R2 R1 Noninverting Amplifier TC7652 TC7652 TC7652 |
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