Datasheet
INP
INM
R
23W
ROUTING
R
5kW
BIAS
R
5kW
BIAS
R
23W
ROUTING
C
2.5pF
PAD
L
1nH
PKG
L
1nH
PKG
R
200W
PAD
C
2.5pF
PAD
R
200W
PAD
VCM = 1.7V
C
EQ
(1)
R
EQ
(2)
Buffer
Buffer
Sampling
Circuit
C
EQ
R
EQ
C
0.5pF
PIN
C
0.5pF
PIN
ADS41B29
ADS41B49
www.ti.com
SBAS486E – NOVEMBER 2009–REVISED JULY 2012
APPLICATION INFORMATION
THEORY OF OPERATION
The ADS41B49/29 is a family of buffered analog input and ultralow power ADCs with maximum sampling rates
up to 250MSPS. The conversion process is initiated by a rising edge of the external input clock and the analog
input signal is sampled. The sampled signal is sequentially converted by a series of small resolution stages, with
the outputs combined in a digital correction logic block. At every clock edge the sample propagates through the
pipeline, resulting in a data latency of 21 clock cycles. The output is available as 14-bit data or 12-bit data, in
DDR LVDS mode or CMOS mode, and coded in either straight offset binary or binary twos complement format.
ANALOG INPUT
The analog input pins have analog buffers (running off the AVDD_BUF supply) that internally drive the differential
sampling circuit. As a result of the analog buffer, the input pins present high input impedance to the external
driving source (10kΩ dc resistance and 3.5pF input capacitance). The buffer helps to isolate the external driving
source from the switching currents of the sampling circuit. This buffering makes it easy to drive the buffered
inputs compared to an ADC without the buffer.
The input common-mode is set internally using a 5kΩ resistor from each input pin to 1.7V, so the input signal can
be ac-coupled to the pins. Each input pin (INP, INM) must swing symmetrically between (VCM + 0.375V) and
(VCM – 0.375V), resulting in a 1.5V
PP
differential input swing.
The input sampling circuit has a high 3dB bandwidth that extends up to 800MHz (measured from the input pins
to the sampled voltage). Figure 59 shows an equivalent circuit for the analog input.
(1) C
EQ
refers to the equivalent input capacitance of the buffer = 4pF.
(2) R
EQ
refers to the R
EQ
buffer = 10Ω.
(3) This equivalent circuit is an approximation and valid for frequencies less than 700MHz.
Figure 59. Analog Input Equivalent Circuit
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Product Folder Link(s): ADS41B29 ADS41B49