Data Sheet

AD8209A

Rev. A | Page 13 of 15

GAIN TRIM

Figure 31 shows a method for incremental gain trimming by

The following approximation is useful for small gain ranges:

For example, using this equation, the adjustment range is ±2%

Figure 31. Incremental Gain Trimming

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

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

where G is the overall gain.

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 can have PARs of 10 or more.

When implementing a filter, consider the PAR so that the output of

the AD8209A preamplifier (A1) does not clip before A2; otherwise,

the nonlinearity is averaged and appears as an error at the output.

To avoid this error, both amplifiers 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, increase the gain of A2 to 5.

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

the features provided by the AD8209A. 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, calculate the gain using the parallel sum

of the resistor and 100 kΩ.

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 is

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

(0.039 µF for a 20 Hz corner frequency).

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. A

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

GND

DNC

–IN

+IN

A1

A2

REXT

OUT

AD8209A

5V

DNC = DO NOT CONNECT

OUTPUT

GAIN TRIM

20kΩ MIN

+

–

+

–

GND

DNC

–IN

+IN

A1

A2

OUT

AD8209A

5V

DNC = DO NOT CONNECT

OUTPUT

1

C IN FARADS

+

–

+

–

GND

DNC

–IN

+IN

A1

A2

OUT

AD8209A

5V

C

C

DNC = DO NOT CONNECT

OUTPUT

255kΩ

+

–

+

–