AD8209

Rev. 0 | Page 13 of 16

Internal Signal Overload Considerations

When configuring the gain for values other than 14, the maximum

input voltage with respect to the supply voltage and ground must

be considered because either the preamplifier or the output buffer

7 for overall gains of ≤7 because the preamplifier, with its fixed

gain of 7 V/V, reaches its full-scale output before the output

buffer. For gains greater than 7, the swing at the buffer output

reaches its full scale first and then limits the AD8209 input to

LOW-PASS FILTERING

In many transducer applications, it is necessary to filter the signal

to remove spurious high frequency components, including noise,

or to extract the mean value of a fluctuating signal with a peak-

to-average ratio (PAR) greater than unity. For example, a full-wave

rectified sinusoid has a PAR of 1.57, a raised cosine has a PAR

of 2, and a half-wave sinusoid has a PAR of 3.14. Signals with

large spikes may have PARs of 10 or more.

When implementing a filter, the PAR should be considered so

that the output of the AD8209 preamplifier (A1) does not clip

before A2; otherwise, the nonlinearity would be averaged and

appear as an error at the output. To avoid this error, both amplifiers

should clip at the same time. This condition is achieved when the

PAR is no greater than the gain of the second amplifier (2 for

the default configuration). For example, if a PAR of 5 is expected,

the gain of A2 should be increased to 5.

Low-pass filters can be implemented in several ways by using

the features provided by the AD8209. In the simplest case, a

single-pole filter (20 dB/decade) is formed when the output

of A1 is connected to the input of A2 via the internal 100 kΩ

resistor by tying Pin 3 to Pin 4 and adding a capacitor from this

node to ground, as shown in Figure 32. If a resistor is added

across the capacitor to lower the gain, the corner frequency

increases; therefore, gain should be calculated using the parallel

sum of the resistor and 100 kΩ.

GND

NC

–IN

+IN

A1

A2

OUT

AD8209

5V

NC = NO CONNECT

OUTPUT

1

2

C IN FARADS

+

–

+

–

Figure 32. Single-Pole, Low-Pass Filter Using the Internal 100 kΩ Resistor

If the gain is raised using a resistor, as shown in Figure 30, the

corner frequency is lowered by the same factor as the gain is raised.

Therefore, using a resistor of 200 kΩ (for which the gain would

be doubled), results in a corner frequency scaled to 0.796 Hz μF

(0.039 μF for a 20 Hz corner frequency).

GND

NC

–IN

+IN

A1

A2

OUT

AD8209

5V

C

C

NC = NO CONNECT

OUTPUT

255k

Ω

+

–

+

–

Figure 33. Two-Pole, Low-Pass Filter

A two-pole filter with a roll-off of 40 dB/decade can be

implemented using the connections shown in Figure 33. This

configuration is a Sallen-Key form based on a ×2 amplifier. It is

useful to remember that a two-pole filter with a corner frequency

Using the standard resistor value shown in Figure 33 and capacitors

of equal values, the corner frequency is conveniently scaled to

1 Hz μF (0.05 μF for a 20 Hz corner frequency). A maximal flat

response occurs when the resistor is lowered to 196 kΩ, scaling

the corner frequency to 1.145 Hz μF. The output offset is raised

by approximately 5 mV (equivalent to 250 μV at the input pins).

FREQUENCY

20dB/DECADE

40dB/DECADE

Figure 34. Comparative Responses of Single-Pole and Two-Pole Low-Pass Filters