DAC1280 datasheet(16/24 Pages) TI

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Time (50 s/div)

LARGE-SIGNAL STEP RESPONSE

OUT

Input Step

125

120

115

110

105

100

-36

-30

-24

-18

-12

-6

Gain (dB)

SNR vs OUTPUT AMPLITUDE

= 2kW

I/V

= 1kW

I/V

= 500W

I/V

Output Frequency (kHz)

CLK = 4.096MHz

Excludes I/V Filter Rolloff

OUTPUT FREQUENCY RESPONSE

DAC1280

SBAS432A – APRIL 2010 – REVISED SEPTEMBER 2010

www.ti.com

If desired, SNR can be improved by decreasing the

settling, the I/V filter network is also settling. The

I/V

feedback

resistors

and

then

applying

suggested I/V RC components (R = 2k

Ω, C = 1nF)

correspondingly higher DAC1280 gains. Decreasing

result in an I/V time constant of approximately 2ms.

the resistor values results in a decrease of the

Figure 32 shows the composite step response of the

maximum output amplitude as shown in Figure 31,

circuit in Figure 35.

SNR versus output amplitude for I/V resistor values of

Ω, 1kΩ, and 500Ω. If decreasing the I/V feedback

resistor, increase the I/V capacitor proportionally to

maintain the same low-pass corner frequency.

Figure 32. DAC1280 Large-Signal Step Response

(Noise Removed for Clarity)

Frequency Response

Figure 31. SNR vs Output Amplitude

The DAC1280 low-pass filters the bitstream input,

resulting in a sinc2 frequency response profile with

the first notch (zero) located at fCLK/160 (25.6kHz with

DC Noise

CLK = 4.096MHz). However, the aspect of noise

shaping of the digital modulator may result in rising

DC noise is measured by the ADS1282 with the

noise versus frequency. This rising noise may limit

circuit configuration of Figure 35. The measurement

the usable bandwidth to less than the DAC inherent

bandwidth is 413Hz and the ADC is set to a

bandwidth.

Figure

illustrates

the

DAC1280

complementing gain. The measurement is taken with

frequency response.

a 50% 1s density input that results in a 0V differential

signal output. DC noise is the standard deviation

(RMS, referred to output).

Total Harmonic Distortion (THD)

The DAC1280 achieves excellent THD performance.

THD was characterized using the circuit shown in

Figure 35 and the ADS1282 with complementary

ADC gain settings for each DAC gain. Note that a

low-distortion

amp

for

current-to-voltage

conversion (such as the OPA211) is essential in

order to achieve rated performance.

Settling Time

The settling time of the DAC1280 resulting from a

step input change consists of the DAC1280 settling

time and the I/V filter settling time. Other filter

components used in the DAC signal path may also

Figure 33. Output Frequency Response

add to the settling time.

When a step input is applied to TDATA, the DAC

output begins to change. The DAC completely settles

in 78ms (CLK = 4.096MHz). As the DAC output is

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