MXO
AINP
Ch0
Chn
Ch1
ADC
GPIO 0
GPIO 1
SDI
GPIO 2
SCLK
CS
SDO
REF
10 F
P
REF5025
o/p
See
Note A
GPIO 3
150 pF
150 pF
R
SOURCE
R
SOURCE
150 pF
R
SOURCE
To
Host
5V
+
-
150pF
100
OPA192
50
ADS7950, ADS7951, ADS7952, ADS7953, ADS7954, ADS7955
ADS7956, ADS7957, ADS7958, ADS7959, ADS7960, ADS7961
SLAS605C – JUNE 2008 – REVISED JULY 2018
Product Folder Links:
ADS7950 ADS7951 ADS7952 ADS7953 ADS7954 ADS7955 ADS7956 ADS7957 ADS7958
Copyright © 2008–2018, Texas Instruments Incorporated
Typical Applications (continued)
9.2.2 OPA192 Buffered Multiplexer Output (MXO)
The use of a buffer relaxes the R
SOURCE
requirements to an extent. Charge from the sample-and-hold capacitor
no longer dominates as a residual charge from a previous channel. Although having good performance is
possible with a larger impedance using the
, the output capacitance of the MXO also holds the previous
channel charge and cannot be isolated, which limits how large the input impedance can finally be for good
performance. In this configuration, the 1xV
REF
range allows slightly higher impedance because the OPA192
(20 V/µs) slews approximately 2.5 V in contrast to the 2xV
REF
range that requires the OPA192 to slew
approximately 5 V.
A.
Restriction on the source impedance exists. R
(SOURCE)
≤
500
Ω
for a 12-bit settling at 1 MSPS with both 1xV
REF
and
2xV
REF
ranges.
Figure 66. Application Diagram for an OPA192 Buffered MXO
9.2.2.1 Design Requirements
The design is optimized to show the input source impedance (R
SOURCE
) from the 100
Ω
to 10000
Ω
required to
meet a 1-LSB settling at 12-bit, 10-bit, and 8-bit resolutions at different throughput in 1xV
REF
(2.5 V) and 2xV
REF
(5 V) input ranges.
9.2.2.2 Detailed Design Procedure
The design procedure is similar to the unbuffered-MXO application, but includes an operation amplifier in unity
gain as a buffer. The most important parameter for multiplexer buffering is slew rate. The amplifier must finish
slewing before the start of sampling (acquisition) to keep the buffer operating in small-signal mode during
sampling (acquisition) time. Also, between the buffer output and converter input (INP), there must be a capacitor
large enough to keep the buffer in small-signal operation during sampling (acquisition) time. Because 150 pF is
large enough to protect the buffer form hold charge from internal capacitors, this value selected along with the
lowest impedance that allows the op amp to remain stable.
The converter allows the MXO to settle approximately 600 ns before sampling. During this time, the buffer slews
and then enters small-signal operation. For a 5-V step change, slew rate stays constant during the first 4 V. The
last 1 V includes a transition from slewing and non-slewing. Thus, the buffer cannot be assumed to keep a
constant slew during the 600 ns available for MXO settling. Assuming that the last 1-V slew is reduced to half is
recommended. For this reason, slew is 10 V/µs or (5 V
ref
+ 1 V) / 0.6 µs to account for the 1-V slow slew. The
OPA192 has a 20-V/us slew, and is capable of driving 150 pF with more than a 50° phase margin with a 50-
Ω
or
100-
Ω
R
iso
, making the OPA192 an ideal selection for the ADS79xx-Q1 family of converters.
