Design Example - Analog Devices DC2837A-A Demo Manual

DEMO MANUAL
DC2837A-A/DC2837A-B
OPERATING PRINCIPLES
Negative offsets may also be applied by shifting JP1 and
–
JP3 to tie to V , and populating R10 instead of R9 with
the desired R .
G
SHUTDOWN ENABLE/DISABLE
Jumper JP6 is provided to enable or disable SHDN easily.
To filter noise before it reaches the SHDN pin, 1k R8 and
0.1μF C10 are provided in series.
To enable the part, tie JP6 to 1, EN or leave it floating. To
disable it, tie JP6 to 3, DIS.
To use an external shutdown control signal, remove the
jumper at JP6 completely, and apply the external signal
via the SHDN and GND turrets. Remember to apply an
offset voltage if the part is being used in single-supply
configuration.
If no signal is applied with the jumper at JP6 completely
unstuffed, the part will float to enable by default.
CLOCK FEEDTHROUGH AND INPUT IMPEDANCE
Since the LTC2063/LTC2066 is a zero-drift amplifier, very
large R or R values connected at the inputs can poten-
FB G
tially lead to clock feedthrough appearing in the output,
especially if the gain setting of the amplifier is not high
enough that GBW roll-off will naturally attenuate signals
at the clock frequency.
Placing a capacitor in parallel with a large R
filter out this undesired clock signal. On this board, if R
is R5, a filter capacitor can be stuffed in Z2 if JP5 is set
to connect Z2 and R5 in parallel (default). The filter cre-
ated by R5 and a capacitor in Z2 must roll-off at least a
decade below 5kHz for DC2838A-A (LTC2063) or 25kHz
for DC2837A-B (LTC2066) to be effective.
6

DESIGN EXAMPLE

Figure 7 shows a general textbook second-order active
lowpass filter, which may be either a Bessel or Butterworth
filter depending on the choice of component values. The
equations for this filter are:
ƒ •k =
–3dB
R1
IN
Figure 7. Generic Second-Order Lowpass Filter
can help
FB
FB
Z1
A
=
V
R1
1
2π C1•C2 • Z1•R2
C1• Z1
C2 •R2
Q =
Z1
A
+ +1
V
R2
Z1
C2
R2
C1
R6 R7
0 49.9
OUT
DC2837A-B F07
Rev. 0