AD8555ACP-R2 datasheet(17/28 Pages) AD

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AD8555

Rev. 0 | Page 17 of 28

THEORY OF OPERATION

A1, A2, R1, R2, R3, P1, and P2 form the first gain stage of the

differential amplifier. A1 and A2 are auto-zeroed op amps that

minimize input offset errors. P1 and P2 are digital potentiome-

ters, guaranteed to be monotonic. Programming P1 and P2

allows the first stage gain to be varied from 4.0 to 6.4 with 7-bit

resolution (see Table 6 and Equation 3), giving a fine gain

adjustment resolution of 0.37%. R1, R2, R3, P1, and P2 each

have a similar temperature coefficient, so the first stage gain

temperature coefficient is lower than 100 ppm/°C.

A3, R4, R5, R6, R7, P3, and P4 form the second gain stage of the

differential amplifier. A3 is also an auto-zeroed op amp that

minimize input offset errors. P3 and P4 are digital potentiome-

ters, allowing the second stage gain to be varied from 17.5 to

200 in eight steps (see Table 7); they allow the gain to be varied

over a wide range. R4, R5, R6, R7, P3, and P4 each have a similar

temperature coefficient, so the second stage gain temperature

coefficient is lower than 100 ppm/°C.

RF together with an external capacitor connected between

FILT/DIGOUT and VSS or VDD form a low-pass filter. The

filtered signal is buffered by A4 to give a low impedance output

at VOUT. RF is nominally 16 kΩ, allowing a 1 kHz low-pass

filter to be implemented by connecting a 10 nF external

capacitor between FILT/DIGOUT and VSS or between

FILT/DIGOUT and VDD. If low-pass filtering is not needed,

then the FILT/DIGOUT pin must be left floating.

A5 implements a voltage buffer, which provides the positive

supply to the amplifier output buffer A4. Its function is to limit

VOUT to a maximum value, useful for driving analog-to-digital

converters (ADC) operating on supply voltages lower than

VDD. The input to A5, VCLAMP, has a very high input resis-

tance. It should be connected to a known voltage and not left

floating. However, the high input impedance allows the clamp

voltage to be set using a high impedance source, e.g., a potential

divider. If the maximum value of VOUT does not need to be

limited, VCLAMP should be connected to VDD.

A4 implements a rail-to-rail input and output unity-gain volt-

age buffer. The output stage of A4 is supplied from a buffered

version of VCLAMP instead of VDD, allowing the positive

swing to be limited. The maximum output current is limited

between 5 mA to 10 mA.

An 8-bit digital-to-analog converter (DAC) is used to generate a

variable offset for the amplifier output. This DAC is guaranteed

to be monotonic. To preserve the ratiometric nature of the input

signal, the DAC references are driven from VSS and VDD, and

the DAC output can swing from VSS (Code 0) to VDD (Code

255). The 8-bit resolution is equivalent to 0.39% of the differ-

ence between VDD and VSS, e.g., 19.5 mV with a 5 V supply.

The DAC output voltage (VDAC) is given approximately by

() VSS

VSS

VDD

Code

VDAC

−

⎟

⎠

⎞

⎜

⎝

⎛

≈

256

(1)

The temperature coefficient of VDAC is lower than

200 ppm/°C.

The amplifier output voltage (VOUT) is given by

(

) VDAC

VNEG

VPOS

GAIN

VOUT

−

(2)

where GAIN is the product of the first and second stage gains.

VDD

DAC

VSS

VDD

VSS

VDD

VSS

VCLAMP

VPOS

VSS

FILT/

DIGOUT

VOUT

VDD

VSS

VNEG

Figure 49. AD8555 Functional Schematic

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