10
LTC1562
1562fa
Because 2nd order sections with Q ≥ 1 have response
peaks near f
O
, the gain ratio above implies some rules of
thumb:
f
O
< 100kHz ⇒ V2 tends to have the larger swing
f
O
> 100kHz ⇒ V1 tends to have the larger swing.
The following situations are convenient because the
relative swing issue does not arise. The unused output’s
swing is naturally the smaller of the two in these cases:
Lowpass response (resistor input, V2 output, Figure 5)
with f
O
< 100kHz
Bandpass response (capacitor input, V2 output, Figure
6b) with f
O
< 100kHz
Bandpass response (resistor input, V1 output, Figure
6a) with f
O
> 100kHz
Highpass response (capacitor input, V1 output, Figure
7) with f
O
> 100kHz
The LTC1562-2, a higher frequency derivative of the
LTC1562, has a design center f
O
of 200kHz compared to
100kHz in the LTC1562. The rules summarized above
apply to the LTC1562-2 but with 200kHz replacing the
100kHz limits. Thus, an LTC1562-2 lowpass filter section
with f
O
below 200kHz automatically satisfies the desirable
condition of the unused output carrying the smaller signal
swing.
APPLICATIONS INFORMATION
WUU
U
level inputs require further dynamic range, reducing the
value of Z
IN
boosts the signal gain while reducing the input
referred noise. This feature can increase the SNR for low
level signals. Varying or switching Z
IN
is also an efficient
way to effect automatic gain control (AGC). From a system
viewpoint, this technique boosts the ratio of maximum
signal to minimum noise, for a typical 2nd order lowpass
response (Q = 1, f
O
= 100kHz), to 118dB.
Input Voltages Beyond the Power Supplies
Properly used, the LTC1562 can accommodate input
voltage excursions well beyond its supply voltage. This
requires care in design but can be useful, for example,
when large out-of-band interference is to be removed from
a smaller desired signal. The flexibility for different input
voltages arises because the INV inputs are at virtual
ground potential, like the inverting input of an op amp with
negative feedback. The LTC1562 fundamentally responds
to input
current
and the external voltage V
IN
appears only
across the external impedance Z
IN
in Figure 3.
To accept beyond-the-supply input voltages, it is impor-
tant to keep the LTC1562 powered on, not in shutdown
mode, and to avoid saturating the V1 or V2 output of the
2nd order section that receives the input. If any of these
conditions is violated, the INV input will depart from a
virtual ground, leading to an overload condition whose
recovery timing depends on circuit details. In the event
that this overload drives the INV input beyond the supply
voltages, the LTC1562 could be damaged.
The most subtle part of preventing overload is to consider
the possible input signals or spectra and take care that
none of them can drive either V1 or V2 to the supply limits.
Note that neither output can be allowed to saturate, even
if it is not used as the signal output. If necessary the
passband gain can be reduced (by increasing the imped-
ance of Z
IN
in Figure 3) to reduce output swings.
The final issue to be addressed with beyond-the-supply
inputs is current and voltage limits. Current entering the
virtual ground INV input flows eventually through the
output circuitry that drives V1 and V2. The input current
magnitude (V
IN
/Z
IN
in Figure 3) should be limited by
design to less than 1mA for good distortion performance.
On the other hand, the input voltage V
IN
appears across the
Low Level or Wide Range Input Signals
The LTC1562 contains a built-in capability for low noise
amplification of low level signals. The Z
IN
impedance in
each 2nd order section controls the block’s gain. When set
for unity passband gain, a 2nd order section can deliver an
output signal more than 100dB above the noise level. If low
Figure 8. 100kHz, Q = 0.7 Lowpass Circuit for
Distortion vs Loading Test
INV V1
2nd ORDER
1/4 LTC1562
V2
1562 F08
R2
10k
C
L
30pF
R
L
(EXTERNAL
LOAD RESISTANCE)
R
Q
6.98k
R
IN
10k
V
IN
V
OUT
