Datasheet

Texas Instruments Incorporated
Amplifiers: Op Amps
26
Analog Applications Journal
Analog and Mixed-Signal Products www.ti.com/sc/analogapps 3Q 2003
Although all of these ferrite chips have
the same impedance at 100 MHz (600 Ω),
they produced different results. The HD
series high-Q chip shows a very narrow
and large peak that will most likely result
in instability and oscillations. The AG and
HG series low-Q chips both performed
about the same, and either one would
probably produce acceptable results. The
only difference is that the HG series has
impedance at higher frequencies and
would probably be better suited for use
with very high-speed CFB amplifiers such
as the OPA685 or the THS3202.
Notice that the pure resistance has a
lower response peak than the ferrite chips.
Coupled with the fact that the HD series
has a high Q and a high peak, this implies
that the slope of the impedance at the
amplifier’s bandwidth is a factor for stabil-
ity. This makes a lot of sense; as it is well
known that for any amplifier, if a zero
intersects the amplifier’s open-loop
response at a rate of closure of 40 dB/
decade, large peaking and oscillations will
most likely result.
5
For this circuit config-
uration, if the impedance of Z has a large
slope that intersects the transimpedance
curve at essentially a rate of closure of
40 dB/decade, peaking and oscillations
also will most likely occur. By comparison,
a resistor intersects the transimpedance
curve at a rate of closure of 20 dB/decade,
resulting in a stable response. Even though
the low-Q ferrite beads have some slope
related to their impedance, the rate of
closure is much lower than 40 dB/decade,
providing improved stability. Nevertheless,
minimizing this intersection rate of closure
as much as possible should produce
acceptable results.
To further expand on the usefulness of
the ferrite chips, more testing was done
utilizing the AG series in the circuit, as
shown in Figure 6.
This figure shows that, just like the
results for the pure resistor, the higher
the impedance is, the lower the peaking.
How does this affect the output noise of
the system? Figure 7 shows the output
noise when the ferrite chips were used,
along with the output noise of the THS4012
and some of the original resistor configurations.
As expected, due to the low frequency impedance of the
ferrite chips, the noise is extremely low. This noise was the
same regardless of which ferrite was used. If noise above
10 MHz was important, the impedance of these ferrite
chips would start to increase the output noise to the same
extent as resistors. These tests show that there are several
advantages of using ferrite chips over resistors.
Inverting gain configuration
All of the testing discussed so far was done with the non-
inverting gain configuration. This configuration forces the
inverting node voltage to move proportionally to the input
voltage applied. So how does the system work in the
inverting gain configuration where the inverting node is
held at a virtual ground? The easy answer is that it works
10 k 100 k 1 M 10 M 100 M 1 G
Frequency (Hz)
V (dB)
OUT
30
25
20
15
10
5
0
–5
–10
–15
xxx = 221
xxx = 471
xxx = 601
xxx = 102
Z = Ferrite Chip
BLM18AGxxxSN1 Series
Figure 6. Responses with AG series ferrite chips (gain = +5)
Z = 681 Ω
Z = All Ferrite Chips
THS4012
Z = 332 Ω
10 k 100 k 1 M 10 M
Frequency (Hz)
Output Noise (nV/ Hz)√
—–
50
45
40
35
30
25
20
15
10
5
0
Figure 7. Output noise comparison (gain = +5)