REV. B

AD8200

–7–

Internal Signal Overload Considerations

When configuring gain for values other than 20, the maximum

input voltage with respect to the supply voltage and ground

must be considered, since either the preamplifier or the output

with large differential input voltages. The input of the AD8200

÷ 10, for overall gains ≤10, since 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

÷ G, 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 having large spikes may have PARs of 10 or more.

When implementing a filter, the PAR should be considered so

the output of the AD8200 preamplifier (A1) does not clip before

A2, since this nonlinearity would be averaged and appear as an

error at the output. To avoid this error, both amplifiers should

be made to clip at the same time. This condition is achieved

when the PAR is no greater than the gain of the second ampli-

fier (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 using the

features provided by the AD8200. 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 k

Ω resistor by

strapping Pins 3 and 4 and a capacitor added from this node to

ground, as shown in Figure 8. If a resistor is added across the

capacitor to lower the gain, the corner frequency will increase; it

should be calculated using the parallel sum of the resistor and

100 k

Ω.

5V

C

OUT

1

C IN FARADS

2

2

NC = NO CONNECT

GND

NC

–IN

+IN

A1

A2

OUT

AD8200

Figure 8. A Single-Pole, Low-Pass Filter Using the

Internal 100 k

Ω Resistor

If the gain is raised using a resistor, as shown in Figure 8, the

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

raised. Thus, using a resistor of 200 k

Ω (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.)

OUT

C

255k

C

5V

NC = NO CONNECT

2

2

GND

NC

–IN

+IN

A1

A2

OUT

AD8200

Figure 9. 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 9. This is a Sallen-Key

form based on a

×2 amplifier. It is useful to remember that a 2-pole

in Figure 10. Using the standard resistor value shown and equal

capacitors (Figure 9), 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 k

Ω and the scaling is

then 1.145 Hz-

µF. The output offset is raised by approximately

5 mV (equivalent to 250 V at the input pins).

1

FREQUENCY

1

20dB/DECADE

40dB/DECADE

Figure 10. Comparative Responses of 1-Pole and

2-Pole Low-Pass Filters