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IVC102P Datasheet(PDF) 6 Page - Burr-Brown (TI) |
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IVC102P Datasheet(HTML) 6 Page - Burr-Brown (TI) |
6 / 10 page 6 ® IVC102 C INT for constant I IN, at the end of TINT V O = –IIN T INT C INT V O I IN I IN(t) V O = –1 ∫ dt C INT In addition, the offset voltage of the internal op amp and charge injection of S2 contribute to the voltage on CINT at the start of integration. Performance of this basic approach can be improved by sampling VO after the reset period at T1 and subtracting this measurement from the final sample at T 2. Op amp offset voltage, charge injection effects and I•RS2 offset voltage on S2 are removed with this two-point measurement. The effec- tive integration period is the time between the two measure- ments, T 2-T1. COMPARISON TO CONVENTIONAL TRANSIMPEDANCE AMPLIFIERS With the conventional transimpedance amplifier circuit of Figure 2a, input current flows through the feedback resistor, RF, to create a proportional output voltage. VO = –IIN RF The transimpedance gain is determined by R F. Very large values of R F are required to measure very small signal current. Feedback resistor values exceeding 100M Ω are common. The IVC102 (Figure 2b) provides a similar function, converting an input current to an output voltage. The input current flows through the feedback capacitor, CINT, charging it at a rate that is proportional to the input current. With a constant input current, the IVC102’s output voltage is VO = –IIN TINT/CINT after an integration time of TINT. VO is proportional to the integration time, TINT, and inversely proportional to the feedback capacitor, CINT. The effective transimpedance gain is TINT /CINT. Ex- tremely high gain that would be impractical to achieve with a conventional transimpedance amplifier can be achieved with small integration capacitor values and/or long integration times. For example the IVC102 with CINT = 100pF and TINT = 100ms provides an effective transimpedance of 1G Ω. A 10nA input current would produce a 10V output after 100ms integration. The integrating behavior of the IVC102 reduces noise by averaging the input noise of the sensor, amplifier, and external sources. Conventional Transimpedance Amplifier Figure 2a Integrating Transimpedance Amplifier Figure 2b R F V O = –IIN RF V O I IN CURRENT-OUTPUT SENSORS Figure 3 shows a model for many current-output sensors such as photodiodes and ionization chambers. Sensor output is a signal-dependent current with a very high source resis- tance. The output is generally loaded into a low impedance FIGURE 2. Comparison to a Conventional Transimpedance Amplifier. so that the terminal voltage is kept very low. Typical sensor capacitance values range from 10pF to over 100pF. This capacitance plays a key role in operation of the switched- input measurement technique (see next section). Provides time-continuous output voltage proportional to IIN. Output voltage after integration period is proportional to average IIN throughout the period. |
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