OPA659 Datasheet, PDF (16/35 Pages) Texas Instruments – Wideband, Unity-Gain Stable, JFET-Input OPERATIONAL AMPLIFIER

OPA659
SBOS342C – DECEMBER 2008 – REVISED NOVEMBER 2015
www.ti.com
The inverting circuit can also use a wide range of resistor values to set the gain; Table 3 lists several
recommended resistor values for inverting gains with a 50-Ω input and output match.
Table 3. Resistor Values For Inverting Gains With 50-Ω Input/Output Match
INVERTING GAIN
R_F
–1
249
–2
249
–5
249
–10
499
R_G
R_T
249
61.9
124
84.5
49.9
Open
49.9
Open
R_OUT
49.9
49.9
49.9
49.9
Figure 37 shows the noninverting input tied directly to ground. Often, a bias current-cancelling resistor to ground
is included here to nullify the DC errors caused by input bias current effects. For a JFET input op amp such as
the OPA659, the input bias currents are so low that dc errors caused by input bias currents are negligible. Thus,
TI does not recommend a bias current-cancelling resistor at the noninverting input.
9.1.3 Operating Suggestions
9.1.3.1 Setting Resistor Values To Minimize Noise
The OPA659 provides a very low input noise voltage. To take full advantage of this low input noise, designers
must pay careful attention to other possible noise contributors. Figure 38 shows the op amp noise analysis model
with all the noise terms included. In this model, all the noise terms are taken to be noise voltage or current
density terms in either nV/√Hz or pA/√Hz.
e_N
R_T
I_BN
OPA659
e_O
4kTR_T
I_BI
4kT
R_G
R_F
4kTR_F
R_G
Figure 38. Op Amp Noise Analysis Model
The total output spot noise voltage can be computed as the square root of the squared contributing terms to the
output noise voltage. This computation adds all the contributing noise powers at the output by superposition, then
takes the square root to arrive at a spot noise voltage. Equation 1 shows the general form for this output noise
voltage using the terms shown in Figure 38.
2
[ ] ^eO ⁼
4kTR_T + (I_BNR_T)² + e_N2
1
+
R_F
R_G
+ (I_BIR_F)² + 4kTR_F
1
+
R_F
R_G
(1)
Dividing this expression by the noise gain (G_N = 1 + R_F/R_G) gives the equivalent input-referred spot noise voltage
at the noninverting input, as Equation 2 shows.
2
e_NI =
4kTR_T + (I_BNR_T)² + e_N2 +
I_BIR_F
Noise Gain
+
4kTR_F
Noise Gain
(2)
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