LT1792 Linear Technology, LT1792 Datasheet - Page 9

• Manufacturer Part Number: LT1792
• Description: Low Noise/ Precision/ JFET Input Op Amp
• Manufacturer: Linear Technology

the input of the LT1792. The charge across the transducer
capacitance, C
C
The gain therefore is C
equal the transducer capacitance plus the input capaci-
tance of the LT1792 and R
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 R
either the transducer impedance or an external resistor.
Since R
than the parallel combination of R1 and R2, R
balance the DC offset caused by the noninverting input
bias current and R
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 R
determined by equating the thermal noise (4kTR
current noise times R
R
is used to cancel the phase shift caused by the op amp
input capacitance and R
Reduced Power Supply Operation
The LT1792 can be operated from 5V supplies for lower
power dissipation resulting in lower I
A
F
B
, resulting in a change in voltage, dV, equal to dQ/C
PPLICATI
= 2V
T
S
/I
is usually several orders of magnitude greater
B
(V
T
S
= 26mV at 25 C). A parallel capacitor, C
, is transferred to the feedback capacitor
O
INPUT: 5.2V Sine Wave
S
U
. The input bias currents, although
Figure 4. Voltage Follower with Input Exceeding the Common Mode Range ( V
S
F
/C
B
S
.
I FOR ATIO
S
, [(2qI
U
. For unity gain, the C
F
should equal R
B
) • R
W
B
S
and noise at the
], resulting in
1792 F04a
B
S
is added to
, which is
S
U
F
S
. In the
) to the
should
S
is
B
F
.
,
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
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 (A
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
JA
of 190 C/W, which results in a die temperature in-
LT1792 Output
S
= 5V)
1792 F03b
LT1792
V
= 1)
9