AD8203YRZ-R7 datasheet(15/20 Pages) AD

15 / 20 page

AD8203

Rev. B | Page 15 of 20

GAIN TRIM

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

the features provided by the AD8203. 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 strapping Pin 3, Pin 4, and a capacitor added from

this node to ground, as shown in

Figure 45 shows a method for incremental gain trimming by

using a trim potentiometer and external resistor R

EXT

The following approximation is useful for small gain ranges:

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.

ΔG ≈ (10 MΩ/REXT)%

Thus, the adjustment range is ±2% for REXT = 5 MΩ; ±10% for

REXT = 1 MΩ, and so on.

VCM

VDIFF

NC = NO CONNECT

GND

–IN

+IN

+VS

OUT

AD8203

OUTPUT

fC =

πC105

C IN FARADS

OUT

REXT

GAIN TRIM

20k

Ω MIN

VCM

VDIFF

NC = NO CONNECT

GND

–IN

+IN

+VS

OUT

AD8203

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

Figure 45. Incremental Gain Trim

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).

Internal Signal Overload Considerations

When configuring gain for values other than 14, the maximum

input voltage with respect to the supply voltage and ground

must be considered, since either the preamplifier or the output

buffer reaches its full-scale output (approximately VS − 0.2 V)

with large differential input voltages. The input of the AD8203

is limited to (V

VCM

VDIFF

NC = NO CONNECT

GND

–IN

+IN

+VS

OUT

AD8203

OUT

255k

fC(Hz) = 1/C(μF)

− 0.2)/7 for overall gains ≤ 7, since the pre-

amplifier, with its fixed gain of ×7, 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 limits the

AD8203 input to (VS − 0.2)/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, includ-

ing 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.

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 that a 2-pole

filter with a corner frequency f2 and a 1-pole filter with a corner at f1

have the same attenuation at the frequency (f

When implementing a filter, the PAR should be considered so

that the output of the AD8203 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 sec-

ond amplifier (2 for the default configuration). For example, if a

PAR of 5 is expected, the gain of A2 should be increased to 5.

2/f1). The attenuation

at that frequency is 40 log (f2/f1), which is 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).

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