IVC102 Datasheet, PDF (6/10 Pages) Burr-Brown (TI) – PRECISION SWITCHED INTEGRATOR TRANSIMPEDANCE AMPLIFIER

In addition, the offset voltage of the internal op amp and
charge injection of S₂ contribute to the voltage on C_INT at the
start of integration.
Performance of this basic approach can be improved by
sampling V_O after the reset period at T₁ and subtracting this
measurement from the final sample at T₂. Op amp offset
voltage, charge injection effects and I•R_S2 offset voltage on
S₂ are removed with this two-point measurement. The effec-
tive integration period is the time between the two measure-
ments, T₂-T₁.
COMPARISON TO CONVENTIONAL TRANSIMPEDANCE AMPLIFIERS
With the conventional transimpedance amplifier circuit
of Figure 2a, input current flows through the feedback
resistor, R_F, to create a proportional output voltage.
V_O = –I_IN R_F
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 100MΩ 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, C_INT,
charging it at a rate that is proportional to the input
current. With a constant input current, the IVC102’s
output voltage is
V_O = –I_IN T_INT/ C_INT
after an integration time of T_INT.
Conventional Transimpedance Amplifier
Figure 2a
V_O is proportional to the integration time, T_INT, and
inversely proportional to the feedback capacitor, C_INT.
The effective transimpedance gain is T_INT /C_INT. 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
C_INT = 100pF and T_INT = 100ms provides an effective
transimpedance of 1GΩ. 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.
Integrating Transimpedance Amplifier
Figure 2b
I_IN
R_F
I_IN
C_INT
V_O
V_O = –I_IN R_F
Provides time-continuous output
voltage proportional to I_IN.
V_O
∫ ^V
O⁼
–1
C_INT
I_IN(t) dt
for constant I_IN, at the end of T_INT
V_O = –I_IN
T_INT
C_INT
Output voltage after integration period is
proportional to average I_IN throughout
the period.
FIGURE 2. Comparison to a Conventional Transimpedance Amplifier.
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
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).
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IVC102
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