LT1113_15 Datasheet, PDF (9/16 Pages) Linear Technology – Dual Low Noise, Precision, JFET Input Op Amp

LT1113
APPLICATI S I FOR ATIO
The LT1113 dual in the plastic and ceramic DIP packages
are pin compatible with and directly replace such JFET op
amps as the OPA2111 and OPA2604 with improved noise
performance. Being the lowest noise dual JFET op amp
available to date, the LT1113 can replace many bipolar op
amps that are used in amplifying low level signals from
high impedance transducers. The best bipolar op amps
will eventually loose out to the LT1113 when transducer
impedance increases due to higher current noise. The low
voltage noise of the LT1113 allows it to surpass every dual
and most single JFET op amps available. For the best
performance versus area available anywhere, the LT1113
is offered in the narrow SO-8 surface mount package with
standard pinout and no degradation in performance.
The low voltage and current noise offered by the LT1113
makes it useful in a wide range of applications, especially
where high impedance, capacitive transducers are used
such as hydrophones, precision accelerometers and photo
diodes. The total output noise in such a system is the gain
times the RMS sum of the op amp input referred voltage
noise, the thermal noise of the transducer, and the op amp
bias current noise times the transducer impedance.
Figure 1 shows total input voltage noise versus source
resistance. In a low source resistance (<5k) application
the op amp voltage noise will dominate the total noise.
This means the LT1113 will beat out any dual JFET op amp,
only the lowest noise bipolar op amps have the edge
(at low source resistances). As the source resistance
increases from 5k to 50k, the LT1113 will match the best
bipolar op amps for noise performance, since the thermal
noise of the transducer (4kTR) begins to dominate the
total noise. A further increase in source resistance, above
50k, is where the op amp’s current noise component (2qI_B
R_TRANS) will eventually dominate the total noise. At these
high source resistances, the LT1113 will out perform
the lowest noise bipolar op amp due to the inherently low
current noise of FET input op amps. Clearly, the LT1113
will extend the range of high impedance transducers
that can be used for high signal to noise ratios. This
makes the LT1113 the best choice for high impedance,
capacitive transducers.
The high input impedance JFET front end makes the
LT1113 suitable in applications where very high charge
sensitivity is required. Figure 2 illustrates the LT1113 in its
inverting and noninverting modes of operation. A charge
amplifier is shown in the inverting mode example; here the
gain depends on the principal of charge conservation at
the input of the LT1113. The charge across the transducer
capacitance, C_S, is transferred to the feedback capacitor
C_F, resulting in a change in voltage, dV, equal to dQ/C_F.
1k
LT1124*
C_S
LT1113*
R_S
100
–
+
V_O
R_S CS
LT1124^†
10
LT1113^†
LT1113
LT1124
RESISTOR NOISE ONLY
1
100 1k 10k 100k 1M 10M 100M
SOURCE RESISTANCE (Ω)
SOURCE RESISTANCE = 2R_S = R
* PLUS RESISTOR
† _{PLUS RESISTOR  1000pF CAPACITOR}
V_n = A_V √V_{n2(OP AMP)} + 4kTR + 2q I_B • R²
1113 • F01
Figure 1. Comparison of LT1113 and LT1124 Total Output 1kHz Voltage Noise Versus Source Resistance
1113fb
9