9
LT1792
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PPLICATI
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I FOR ATIO
the input of the LT1792. The charge across the transducer
capacitance, CS, is transferred to the feedback capacitor
CF, resulting in a change in voltage, dV, equal to dQ/CF.
The gain therefore is CF/CS. For unity gain, the CF should
equal the transducer capacitance plus the input capaci-
tance of the LT1792 and RF should equal RS. In the
noninverting mode example, the transducer current is
converted to a change in voltage by the transducer capaci-
tance; this voltage is then buffered by the LT1792 with a
gain of 1 + R1/R2. A DC path is provided by RS, which is
either the transducer impedance or an external resistor.
Since RS is usually several orders of magnitude greater
than the parallel combination of R1 and R2, RB is added to
balance the DC offset caused by the noninverting input
bias current and RS. The input bias currents, although
small at room temperature, can create significant errors at
higher temperature, especially with transducer resistances
of up to 100M or more. The optimum value for RS is
determined by equating the thermal noise (4kTRS) to the
current noise times RS, [(2qIB) RS], resulting in
RB = 2VT/IB (VT = 26mV at 25°C). A parallel capacitor, CB,
is used to cancel the phase shift caused by the op amp
input capacitance and RB.
Reduced Power Supply Operation
The LT1792 can be operated from
±5V supplies for lower
power dissipation resulting in lower IB and noise at the
expense of reduced dynamic range. To illustrate this
benefit, let’s take the following example:
An LT1792CS8 operates at an ambient temperature of
25
°C with ±15V supplies, dissipating 159mW of power
(typical supply current = 5.3mA). The SO-8 package has a
θJA of 190°C/W, which results in a die temperature in-
crease of 30.2
°C or a room temperature die operating
temperature of 55.2
°C. At ±5V supplies, the die tempera-
ture increases by only one third of the previous amount or
10.1
°C resulting in a typical die operating temperature of
only 35.1
°C. A 20 degree reduction of die temperature is
achieved at the expense of a 20V reduction in dynamic
range.
To take full advantage of a wide input common mode
range, the LT1792 was designed to eliminate phase rever-
sal. Referring to the photographs shown in Figure 4, the
LT1792 is shown operating in the follower mode (AV = 1)
at
±5V supplies with the input swinging ±5.2V. The output
of the LT1792 clips cleanly and recovers with no phase
reversal. This has the benefit of preventing lock-up in
servo systems and minimizing distortion components.
High Speed Operation
The low noise performance of the LT1792 was achieved
by making the input JFET differential pair large to maxi-
mize the first stage gain. Increasing the JFET geometry
INPUT:
±5.2V Sine Wave
LT1792 Output
1792 F04a
1792 F03b
Figure 4. Voltage Follower with Input Exceeding the Common Mode Range ( VS = ±5V)