Circuit Note

CN-0582

analog.com

Rev. A | 7 of 10

WAVEFORM GENERATION AND CONTROL

The CN0582 has an on-board waveform generator used to excite

and control a shaker table, which helps in testing and calibrating

IEPE sensors.

Figure 9 shows the block diagram utilizing the AD9833, a program-

mable waveform generator with a constant voltage output of 600

mVp-p and configurable frequency output from 0 Hz to 12.5 MHz

with 28-bit resolution. The output frequency can be computed using

Equation 10:

FOUT=FREF×FREQ228

(10)

where:

FOUT is the output frequency.

FREF is the reference clock frequency.

FREQ is the frequency register value.

Figure 9. Waveform Generation Block Diagram

The CN0582 frequency output is band limited to 25 kHz, which

is adequate for most vibration and CbM applications. The output

signal is then amplified and conditioned using the ADA4522 in

a high pass filter configuration, which removes the dc offset, a

requirement for many shaker table inputs.

The maximum set gain of ADA4522 can be computed using Equa-

tion 11:

Output_AmplitudeMAX= RfRdiff×600mVp‐p

(11)

Output_AmplitudeMAX=8.16Vp‐p

where:

Output_AmplitudeMAX is the maximum amplitude set by the gain of

the ADA4522.

Rf is 102 kΩ.

Rdiff is 7.5 kΩ.

The signal generator can produce 8.16 Vp-p maximum, but the

graphical user interface (GUI) limits the output voltage to 7 Vp-p.

The CN0582 signal generator output amplitude is controlled by an

AD5543 current output DAC, configured in a current to voltage

topology as shown in Figure 10.

Figure 10. Output Waveform Amplitude Control Block Diagram

The amplitude can be controlled following Equation 12:

Output_Amplitude= D65536×8.16Vp‐p

(12)

where:

Output_Amplitude is the modulated output amplitude.

D is the digital code value.

After configuring the desired output amplitude and frequency, an

optional post-filtering circuit is available to reduce the broadband

noise. The circuit uses the ADA4522 in a Sallen-key low pass filter

configuration at the DAC's output, because of the stability and high

input impedance characteristics. The cutoff frequency is set to 26

kHz, and can be computed using Equation 13:

fc= 1

2π×3.4kΩ×1.8nF

=26.005kHz

(13)

The cutoff frequency is adjusted to at least twice the intended IEPE

bandwidth of 10 kHz, so that negligible attenuation occurs.

The output selector stage shown in Figure 11 uses an ADG1219

switch between filtered and unfiltered output from the amplitude

controller. The selected output is then buffered to isolate the circuit

from the output connector.

Figure 11. Output Waveform Selector Circuit

COMMON VARIATIONS

The AD3552R 16-bit, 33 MSPS DAC is an alternative to the

AD9833 with higher resolution, lower distortion, and arbitrary wave-

form generation capability, however it requires a data source.

The ADG5436F is a fault-protected variant of the ADG5436 that

provides additional protection from input faults, as well as fault

detection.

For the ADC, if the application needs to operate beyond the IEPE

bandwidth, a good alternative is the AD4134. This device has a

higher sampling rate and an improved overall performance.

A higher channel count system can also be realized by using

the AD7768. The existing controller already has a provision for