Datasheet

9
LT1793
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Input: ±5.2V Sine Wave
LT1793 Output
LT1793 F05a LT1793 F05b
Figure 5. Voltage Follower with Input Exceeding the Common Mode Range (V
S
= ±5V)
resulting in a change in voltage dV, which is equal to dQ/C
F
.
The gain therefore is C
F
/C
S
. For unity-gain, the C
F
should
equal the transducer capacitance plus the input capaci-
tance of the LT1793 and R
F
should equal R
S
.
In the noninverting mode example, the transducer current
is converted to a change in voltage by the transducer
capacitance, C
S
. This voltage is then buffered by the
LT1793 with a gain of 1 + R1/R2. A DC path is provided by
R
S
, which is either the transducer impedance or an exter-
nal resistor. Since R
S
is usually several orders of magni-
tude greater than the parallel combination of R1 and R2, R
B
is added to balance the DC offset caused by the noninvert-
ing input bias current and R
S
. The input bias currents,
although small at room temperature, can create significant
errors at higher temperature, especially with transducer
resistances of up to 1000M or more. The optimum value
for R
B
is determined by equating the thermal noise (4kTR
S
)
to the current noise (2qI
B
) times R
S
2
. Solving for R
S
results in R
B
= R
S
= 2V
T
/I
B
(V
T
= 26mV at 25°C). A parallel
capacitor C
B
, is used to cancel the phase shift caused by
the op amp input capacitance and R
B
.
Reduced Power Supply Operation
To take full advantage of a wide input common mode range,
the LT1793 was designed to eliminate phase reversal.
Referring to the photographs in Figure 5, the LT1793 is
shown operating in the follower mode (A
V
= 1) at ±5V
supplies with the input swinging ±5.2V. The output of the
LT1793 clips cleanly and recovers with no phase reversal.
This has the benefit of preventing lockup in servo systems
and minimizing distortion components.