LTC2000_15 Datasheet, PDF (24/52 Pages) Linear Technology – 16-/14-/11-Bit 2.5Gsps DACs

LTC2000
Operation
Table 2. Full-Scale Gain Adjustment
ADDRESS BIT NAME
0x09 [5:0] GAIN_ADJ
GAIN_ADJ (HEX)
0x1F
0x1E
—
0x01
0x00
0x3F
—
0x21
0x20
Note: Register 0x09 resets to 0x00 (default).
DESCRIPTION
GAIN_ADJ (DECIMAL)
+31
+30
—
+1
0
–1
—
–31
–32
GAIN ADJUSTMENT
89.2%
89.5%
—
99.6%
100.0%
100.4%
—
113.8%
114.3%
FULL-SCALE CURRENT
(R_FSADJ = 500Ω, V_REFIO = 1.25V)
35.68mA
35.80mA
—
39.84mA
40.00mA
40.16mA
—
45.51mA
45.71mA
DAC Transfer Function
The LTC2000 contains an array of current sources that
are steered through differential switches to either I_OUTP or
I_OUTN, depending on the DAC code programmed through the
LVDS parallel interface. The LTC2000 uses a 16‑/14‑/11‑bit
two’s complement DAC code. The complementary current
outputs, I_OUTP and I_OUTN, source current from 0mA to
I_OUTFS. For I_OUTFS = 40mA (nominal), I_OUTP swings from
0mA (for zero-scale DAC code) to 40mA (for full-scale
DAC code). I_OUTN is complementary to I_OUTP. When the
DAC code is set to mid-scale (all zeros), I_OUTFS is evenly
divided between I_OUTP and I_OUTN. I_OUTP and I_OUTN are
given by the following formulas:
LTC2000-16:
I_OUTP = I_OUTFS • (CODE + 32768)/65536 + I_OUTCM
I_OUTN = I_OUTFS • (32768 – CODE – 1)/65536 + I_OUTCM
LTC2000-14:
I_OUTP = I_OUTFS • (CODE + 8192)/16384 + I_OUTCM
I_OUTN = I_OUTFS • (8192 – CODE – 1/4)/16384 + I_OUTCM
LTC2000-11:
I_OUTP = I_OUTFS • (CODE + 1024)/2048 + I_OUTCM
I_OUTN = I_OUTFS • (1024 – CODE – 1/32)/2048 + I_OUTCM
The DAC code ranges from –2^N-1 to 2^N-1 – 1, with N
being the DAC resolution (16/14/11). I_OUTCM is a small,
constant common-mode output current that is equal to
approximately 0.2% full-scale, or 80µA for I_OUTFS = 40mA.
The LTC2000 differential output currents typically drive a
resistive load either directly or drive an equivalent resistive
load through a transformer (see the Output Configurations
section). The voltage outputs generated by the I_OUTP and
I_OUTN outputs currents are then:
V_OUTP = I_OUTP • R_LOAD
V_OUTN = I_OUTN • R_LOAD
V_DIFF = V_OUTP – V_OUTN = (I_OUTP-I_OUTN) • R_LOAD
Substituting the values above gives:
LTC2000-16:
V_DIFF = V_REFIO • (R_LOAD/R_FSADJ) • (2 • CODE + 1)/4096
LTC2000-14:
V_DIFF = V_REFIO • (R_LOAD/R_FSADJ) • (2 • CODE + 1/4)/1024
LTC2000-11:
V_DIFF = V_REFIO • (R_LOAD/R_FSADJ) • (2 • CODE + 1/32)/128
Note that the gain of the DAC depends on the ratio of R_LOAD
to R_FSADJ, and the gain error tempco is affected by the
temperature tracking of R_LOAD with R_FSADJ.
Analog Outputs (I_OUTP/N)
The two complementary analog outputs (I_OUTP/N) have low
output capacitance that, with appropriate R_LOAD values,
can achieve high output bandwidths of 2.1GHz. The analog
outputs also have an internal impedance of 50Ω to GND
that will affect the calculation of R_LOAD and the output
2000f
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