Datasheet

AD7242/AD7244
REV. A
–7–
Output Amplifier
The outputs from each of the voltage-mode DACs are buffered
by a noninverting amplifier. The buffer amplifier is capable of
developing ±3 V across a 2 kΩ and 100 pF load to ground, and
can produce 6 V peak-to-peak sine wave signals to a frequency
of 20 kHz. The output is updated on the falling edge of the
respective
LDAC input. The output voltage settling time, to
within 1/2 LSB of its final value, is typically less than 2 µs for
the AD7242 and 2.5 µs for the AD7244.
The small signal (200 mV p-p) bandwidth of the output buffer
amplifier is typically 1 MHz. The output noise from the
amplifier is low, with a figure of 30 nV/√
Hz at a frequency of
1 kHz. The broadband noise from the amplifier exhibits a
typical peak-to-peak figure of 150 µV for a 1 MHz output
bandwidth. Figure 4 shows a typical plot of noise spectral
density versus frequency for the output buffer amplifier and for
the on-chip reference (including and excluding the decoupling
components).
Figure 4. Noise Spectral Density vs. Frequency
TRANSFER FUNCTION
The basic circuit configuration for the AD7242/AD7244 is
shown in Figure 5. Table I and Table II show the ideal input
code to output voltage relationship for the AD7242 and
AD7244 respectively. Input coding for the AD7242/AD7244 is
2s complement.
Figure 5. Basic Connection Diagram
For the AD7242, the output voltage can be expressed in terms
of the input code, N, using the following relationship:
V
OUT
=
2 • N • REF IN
4096
where –2048 ≤ N ≤ +2047
For the AD7244, the output voltage can be expressed in terms
of the input code, N, using the following relationship:
V
OUT
=
2 • N • REF IN
16384
where –8192 ≤ N ≤ +8191
Table I. AD7242 Ideal Input/Output Code Table Code
DAC Latch Contents
MSB LSB Analog Output, V
OUT
*
01 11 1111 1111 +2.998535 V
01 11 1111 1110 +2.99707 V
00 00 0000 0001 +0.001465 V
00 00 0000 0000 0 V
11 11 1111 1111 –0.001465 V
10 00 0000 0001 –2.998535 V
10 00 0000 0000 –3 V
*Assuming REF IN = +3 V.
Table II. AD7244 Ideal Input/Output Code Table Code
DAC Latch Contents
MSB LSB Analog Output, V
OUT
*
01 1111 1111 1111 +2.999634 V
01 1111 1111 1110 +2.99268 V
00 0000 0000 0001 +0.000366 V
00 0000 0000 0000 0 V
11 1111 1111 1111 –0.000366 V
10 0000 0000 0001 –2.999634 V
10 0000 0000 0000 –3 V
*Assuming REF IN = +3 V.