AD8202

Rev. D | Page 15 of 20

GAIN TRIM

Figure 45 shows a method for incremental gain trimming by

The following approximation is useful for small gain ranges:

5V

OUT

GAIN TRIM

20k

Ω MIN

2

2

NC = NO CONNECT

GND

NC

–IN

+IN

A1

A2

OUT

AD8202

Figure 45. Incremental Gain Trim

Internal Signal Overload Considerations

When configuring gain for values other than 20, the maxi-

mum input voltage with respect to the supply voltage and

ground must be considered because either the preamplifier

or the output buffer reaches its full-scale output (approximately

because the preamplifier, with its fixed gain of ×10, reaches its full-

scale output before the output buffer. For gains greater than 10, the

swing at the buffer output reaches its full scale first and limits the

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 having large spikes can have PARs of 10

or more.

When implementing a filter, the PAR should be considered

so that the output of the AD8202 preamplifier (A1) does not

clip before A2 because this 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

AD8202. 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 and Pin 4

and adding a capacitor from this node to ground, as shown in

Figure 46. If a resistor is added across the capacitor to lower the

gain, the corner frequency increases; it should be calculated using

the parallel sum of the resistor and 100 kΩ.

5V

2

2

NC = NO CONNECT

C

GND

NC

–IN

+IN

A1

A2

OUT

AD8202

OUTPUT

1

2

C IN FARADS

Figure 46. Single-Pole, Low-Pass Filter Using the Internal 100 kΩ Resistor

If the gain is raised using a resistor, as shown in Figure 44, the

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

raised. Thus, using a resistor of 200 kΩ (for which the gain

would be doubled), the corner frequency is now 0.796 Hz/μF

(0.039 μF for a 20 Hz corner frequency).

5V

2

2

NC = NO CONNECT

C

GND

NC

–IN

+IN

A1

A2

OUT

AD8202

OUT

C

255k

Ω

Figure 47. 2-Pole, Low-Pass Filter

A 2-pole filter (with a roll-off of 40 dB/decade) can be

implemented using the connections shown in Figure 47. This is a

Sallen-Key form based on a ×2 amplifier. It is useful to remember

illustrated in Figure 48. Using the standard resistor value shown

and equal capacitors (see Figure 47), the corner frequency is

conveniently scaled at 1 Hz/μF (0.05 μF for a 20 Hz corner).

A maximally flat response occurs when the resistor is lowered to

196 kΩ and the scaling is then 1.145 Hz/μF. The output offset

is raised by approximately 5 mV (equivalent to 250 μV at the

input pins).