Rainbow Electronics MAX7480 - Manual (Page 7)

MAX7480
8th-Order, Lowpass, Butterworth,
Switched-Capacitor Filter
_______________________________________________________________________________________ 7
Clock Signal
External Clock
The MAX7480 SCF is designed for use with external
clocks that have a 40% to 60% duty cycle. When using
an external clock with these devices, drive CLK with a
CMOS gate powered from 0 to V
DD . Varying the rate of
the external clock adjusts the corner frequency of the
filter as follows:
f C = f CLK / 100
Internal Clock
When using the internal oscillator, connect a capacitor
(C
OSC ) between CLK and ground. The value of the
capacitor determines the oscillator frequency as follows:
Minimize the stray capacitance at CLK so that it does
not affect the internal oscillator frequency. Vary the rate
of the internal oscillator to adjust the filter’s corner fre-
quency by a 100:1 clock to corner-frequency ratio. For
example, an internal oscillator frequency of 100kHz
produces a nominal corner frequency of 1kHz.
Input Impedance vs. Clock Frequencies
The MAX7480’s input impedance is effectively that of a
switched-capacitor resistor, and is inversely proportion-
al to frequency. The input impedance values deter-
mined below represent the average input impedance,
since the input current is not continuous. As a rule, use
a driver with an output impedance less than 10% of the
filter’s input impedance. Estimate the input impedance
of the filter using the following formula:
where f CLK = clock frequency and C IN = 2.31pF.
Low-Power Shutdown Mode
This device features a shutdown mode that is activated
by driving SHDN low. In shutdown mode, the filter’s sup-
ply current reduces to 0.2µA (typ) and its output
becomes high impedance. For normal operation, drive
SHDN high or connect to V
DD
.
___________Applications Information
Offset and Common-Mode
Input Adjustment
The voltage at COM sets the common-mode input volt-
age and is biased at mid-supply with an internal resis-
tor-divider. Bypass COM with a 0.1µF capacitor and
connect OS to COM. For applications requiring offset
adjustment or DC level shifting, apply an external bias
voltage through a resistor-divider network to OS, as
shown in Figure 3. (Note: Do not leave OS unconnect-
ed.) The output voltage is represented by this equation:
V
OUT
= (V
IN
- V
COM
) + V
OS
with V COM = V DD / 2 (typical), where (V IN - V COM ) is
lowpass-filtered by the SCF and VOS is added at the
output stage. See the
Electrical Characteristics for the
voltage range of COM and OS. Changing the voltage
on COM or OS significantly from mid-supply reduces
the filter’s dynamic range.
Power Supplies
The MAX7480 operates from a single +5V supply.
Bypass V
DD to GND with a 0.1µF capacitor. If dual
supplies (±2.5V) are required, connect COM to system
ground and connect GND to the negative supply.
Figure 4 shows an example of dual-supply operation.
Single- and dual-supply performances are equivalent.
For either single- or dual-supply operation, drive CLK
and SHDN from GND (V- in dual-supply operation) to
V
DD . For ±5V dual-supply applications, use the
MAX291–MAX297.
Input Signal Amplitude Range
The optimal input signal range is determined by observ-
ing the voltage level at which the total harmonic distor-
tion plus noise (THD+N) is minimized for a given corner
frequency. The
Typical Operating Characteristics
shows a graph of the device’s THD+N response as the
input signal’s peak-to-peak amplitude is varied. This
measurement is made with OS and COM biased at mid-
supply.
Z
1
f C
IN
CLK IN
=
( )
⋅
f (kHz)
C
; C in pF
OSC
3
OSC
OSC
=
⋅
53 10
V DD
V SUPPLY
IN
CLK
GND
INPUT
OUTPUT
50k
50k
50k
OUT
0.1 µ F
0.1 µ F
0.1 µ F
CLOCK
SHDN
COM
OS
MAX7480
Figure 3. Offset Adjustment Circuit