THS6012_16 Datasheet, PDF (27/37 Pages) Texas Instruments – 500-mA DUAL DIFFERENTIAL LINE DRIVER

THS6012
www.ti.com
SLOS226F – SEPTEMBER 1998 – REVISED JUNE 2012
GENERAL CONFIGURATIONS
A common error for the first-time CFB user is to create a unity gain buffer amplifier by shorting the output directly
to the inverting input. A CFB amplifier in this configuration oscillates and is not recommended. The THS6012,
like all CFB amplifiers, must have a feedback resistor for stable operation. Additionally, placing capacitors
directly from the output to the inverting input is not recommended. This is because, at high frequencies, a
capacitor has a very low impedance. This results in an unstable amplifier and should not be considered when
using a current-feedback amplifier. Because of this, integrators and simple low-pass filters, which are easily
implemented on a VFB amplifier, have to be designed slightly differently. If filtering is required, simply place an
RC-filter at the noninverting terminal of the operational-amplifier (see Figure 48).
R_G
R_F
V_I
R1
–
+
C1
V_O
ǒ Ǔǒ Ǔ ^VO
VI
+
1
)
RF
RG
1
1 ) sR1C1
f–3dB
+
1
2pR1C1
Figure 48. Single-Pole Low-Pass Filter
If a multiple pole filter is required, the use of a Sallen-Key filter can work very well with CFB amplifiers. This is
because the filtering elements are not in the negative feedback loop and stability is not compromised. Because of
their high slew-rates and high bandwidths, CFB amplifiers can create very accurate signals and help minimize
distortion. An example is shown in Figure 49.
C1
V_I
R1
+
R2
_
C2
R_F
R_G
R1 = R2 = R
C1 = C2 = C
Q = Peaking Factor
(Butterworth Q = 0.707)
f–3dB
+
1
2pRC
( ) ^{RG =}
R_F
1
2– _Q
Figure 49. 2-Pole Low-Pass Sallen-Key Filter
There are two simple ways to create an integrator with a CFB amplifier. The first one shown in Figure 50 adds a
resistor in series with the capacitor. This is acceptable because at high frequencies, the resistor is dominant and
the feedback impedance never drops below the resistor value. The second one shown in Figure 51 uses positive
feedback to create the integration. Caution is advised because oscillations can occur because of the positive
feedback.
R_F
C1
R_G
V_I
–
+
V_O
THS6012
ȡ ȣ _VO
ǒ Ǔ_{ȧȢ ȧȤ VI}
+
RF
RG
S
)
1
RFC1
S
Figure 50. Inverting CFB Integrator
Copyright © 1998–2012, Texas Instruments Incorporated
27