TLV277xEP, TLV277xAEP

FAMILY OF 2.7V HIGHSLEW RATE RAILTORAIL OUTPUT

OPERATIONAL AMPLIFIERS WITH SHUTDOWN

SGLS317A − OCTOBER 2005 − REVISED SEPTEMBER 2007

29

POST OFFICE BOX 655303

APPLICATION INFORMATION

gain calculation

is 0 V − 1.23 V = −1.23 V and the maximum positive swing is 3 V − 1.23 V = 1.77 V. By modeling the shock sensor

as a low impedance voltage source with output of 2.25 mV/g (max) in the x and y axis and 1.70 mV/g (max) in

the z axis, the gain of the circuit is calculated by equation 1.

Gain

+

Output Swing

Sensor Signal

Acceleration

(1)

To avoid saturation of the operational amplifier, the gain calculations are based on the maximum negative swing

of −1.23 V and the maximum sensor output of 2.25 mV/g (x and y axis) and 1.70 mV/g (z axis).

Gain (x, y)

+

* 1.23 V

2.25 mV g

* 50 g

+ 10.9

(2)

Gain (z)

+

–1.23 V

1.70 mV g

–50 g

+ 14.5

(3)

and

By selecting R3 = 10 k

Ω and R4 = 100 kΩ, in the x and y channels, a gain of 11 is realized. By selecting

R3 = 7.5 k

Ω and R4 = 100 kΩ, in the z channel, a gain of 14.3 is realized. The schematic shows the configuration

for either the x or y axis.

bandwidth calculation

To calculate the component values for the frequency shaping characteristics of the signal conditioning circuit,

1 Hz and 500 Hz are selected as the minimum required 3-dB bandwidth.

To minimize the value of the input capacitor (C1) required to set the lower cutoff frequency requires a large value

resistor for R2. A 1-M

Ω resistor is used in this example. To set the lower cutoff frequency, the required capacitor

value for C1 is:

C1

+

1

2

p f

+ 0.159 µF

(4)

Using a value of 0.22

µF, a more common value of capacitor, the lower cutoff frequency is 0.724 Hz.

To minimize the phase shift in the feedback loop caused by the input capacitance of the TLV2772, it is best to

minimize the value of the feedback resistor R4. However, to reduce the required capacitance in the feedback

loop a large value for R4 is required. Therefore, a compromise for the value of R4 must be made. In this circuit,

a value of 100 k

Ω has been selected. To set the upper cutoff frequency, the required capacitor value for C2 is:

C2

+

1

2

p f

+ 3.18 µF

(5)

Using a 2.2-nF capacitor, the upper cutoff frequency is 724 Hz.

R5 and C3 also cause the signal response to roll off. Therefore, it is beneficial to design this roll-off point to begin

at the upper cutoff frequency. Assuming a value of 1 k

Ω for R5, the value for C3 is calculated to be

0.22

µF.