THS4520 Datasheet, PDF (24/37 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER WITH RAIL-TO-RAIL OUTPUTS

THS4520
SLOS503B – SEPTEMBER 2006 – REVISED JULY 2007
and the reduced bandwidth due to increased noise
gain when the circuit is configured for low forward
gain. Note that using noise gain compensation
increases the circuit output noise and decreases the
circuit bandwidth. Compared to the default
configuration (no R_C) using R_C = 200 Ω and R_C =
487 Ω increases the circuit output noise by
approximately 10.9dB and 6dB respectively.
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
0.1
R_C = open
R_C = 200 W
NG = 7
R_C = R_G = 487 W
NG = 4
1
10
100
f - Frequency - MHz
1000
Figure 66. THS4520 EVM Small Signal Response
With and Without Noise Gain Compensation
DC ERRORS IN A FULLY DIFFERENTIAL
AMPLIFIER
Summary
A DC error model of a fully differential voltage
feedback amplifier shown in the following circuit
diagram. The output error has four contributing
factors in this model:
1. Input offset voltage (V_IO).
2. Input offset current (I_IO).
3. Input bias currents (I_IB+, I_IB–) interacting with
mismatched feedback networks.
4. Mismatch between input and output
common-mode voltages interacting with the
mismatched feedback networks.
Source
V_IN+ ^RG2
RF₂
V_ID/2 ⁺
-
V_I ⁺
-
V_ID/2 ⁺
-
V_IN- ^RG1
V_S+
V_IO/2
V_P
I_IB+
V_IO/2
V_N
I_IB-
Ideal
FDA
V_OCM
V_S+
V_OUT-
-
V_OD
+
V_OUT+
RF₁
www.ti.com
When there is no mismatch between the feedback
networks (RF₁ = RF₂ and RG₁ = RG₂) the output
error due to the input offset voltage is given by:
ǒ Ǔ _{DVOD VIO}
+
V_IO
RG ) RF
RG
+
V_IOńb
(5)
where β is often called the feedback factor.
b
+
RG
RG ) RF
(6)
For additional information, see the applications note
Fully Differential Amplifiers (SLOA054).
The output error due to the input offset current is
given by:
ǒ Ǔ _{DVOD IIO + IIORF}
(7)
If there is mismatch (RF₁ ≠ RF₂ or RG₁ ≠ RG₂), then
the output error due to the input bias currents is:
ǒ Ǔ ^{ǒ Ǔ ǒ Ǔ} DV_OD I_IB,
I_IO
+
2 ^IIB
R_EQ1
*
R_EQ2 )
ǒ_{b1 )}
I_IO R_EQ1
b₂^Ǔ
)
R_EQ2
(8)
Where I_IB = (I_IB+ + I_IB–)/2, R_EQ1,2 = RF_1,2 || RG_1,2 and
β_1,2 = RG_1,2/(RG_1,2 + RF_1,2).
There is an additional contribution to the output error
if the input and output common-mode voltages are
mismatched:
ǒ Ǔ _{DVOD VOCM, VICM + 2}
ǒ_VOCM
*
Ǔ _VICM
ǒ_b
ǒ_b
1
1
*
)
b₂^Ǔ
b₂^Ǔ
(9)
Note that this source of output error will be negligible
if the two feedback paths are well matched. The
analysis that leads to the results shown above is
beyond the scope of this section. An applications
note that shows the detailed analysis will be
available in the near future.
DEPENDENCE OF HARMONIC DISTORTION
ON DEVICE OUTPUT SWING AND SIGNAL
FREQUENCY
Typical plots of HD2 or HD3 usually show the
dependence of these parameters upon a single
variable, like frequency, output swing, load, or circuit
gain. Operating conditions of interest are usually
dependent on several variables that are often spread
across several different plots. This forces the
designer to interpolate across several plots in an
attempt to capture the parameters and operating
conditions for his/her application.
Unlike typical plots where HD2 or HD3 is plotted
against a single variable, the plots below show
constant contours of THS4520 HD2 and HD3 plotted
against the joint parameters of device output swing
and signal frequency. These two parameters are of
24
Submit Documentation Feedback