Additional Adc Driver Stage (Stage 3); Voltage Reference; Common-Mode Circuit - Analog Devices EVAL-AD4080-FMCZ User Manual

User Guide
EVALUATION BOARD HARDWARE GUIDE
ADA4932-1
►
Stage bypass
►
No bypass (default)
►
Bypass along with Stage 1 for using the AMC
►

ADDITIONAL ADC DRIVER STAGE (STAGE 3)

Stage 3 is an optional stage consisting of two
A6) high-speed, low distortion op amps. This stage allows users to
achieve the lowest distortion possible; however, at the expense of
higher power consumption. Stage 3 is enabled by default, but this
stage can be disabled to save power.
Figure 5. Stage 3 Simplified Schematic
The following can be configured in this stage:
Stage bypass:
►
Bypass stage.
►
Enable stage (default).
►

VOLTAGE REFERENCE

The AD4080 requires an external 3 V voltage reference. To achieve
the specified performance, a suitable precision, low noise voltage
reference must be used. The AD4080 does include an internal
reference buffer and capacitor, which makes reference selection
easier and eliminates the need for an external buffer.
The following can be configured in this circuit:
Reference selection
►
The LTC6655-3 is the default. The evaluation hardware in-
►
cludes LTC6655-3 (U25) as the primary recommended option,
providing exceptional noise performance (0.1 Hz to 10 Hz
noise specification of 0.25 ppm p-p), combined with an initial
accuracy of 0.025%, and a low temperature drift of 2 ppm/°C.
The LT6657-3 (U36) is also mounted and provided as a
►
second option. See Table 1
references.
Table 1. 3 V Reference Comparison of the LT6657 and LTC6655
Parameter
Accuracy
Temperature Coefficient (ppm/°C)
0.1 Hz to 10 Hz Noise (ppm p-p)
Maximum Load (mA)
Load Regulation (ppm/mA)
Maximum Supply
Shutdown
analog.com
ADA4899-1 (A1 and
for a comparison of recommended
LT6657 LTC6655
0.10% 0.025%
1.5 2
0.5 0.25
±10 ±5
0.7 3
40 V 13.2 V
Yes Yes
Table 1. 3 V Reference Comparison of the LT6657 and LTC6655 (Continued)
Parameter
Reverse Supply Protected
Reverse Output Protected
Current Limit
Thermal Protection
Shunt Mode
Supply Current, I (mA)
S
T
A
100% Tested Temperatures

COMMON-MODE CIRCUIT

The AD4080 includes a common-mode voltage generation feature.
The common-mode voltage is equal to reference voltage (V
and this voltage is provided through the CMO pin of the AD4080.
This feature is generally useful for biasing front-end stages. In this
instance, it is optional to use this feature because the
can use an internal biasing circuit to set the output common mode
to the midpoint of the output clamping pins (−V
The following can be configured in this circuit:
FDA common-mode setting
►
Internal (default): The ADA4945-1 sets the output common-
►
mode level to the output clamps midpoint.
External: Use the CMO voltage provided by the ADC to set
►
the FDA output common-mode level.
Common-mode signal buffering
►
No buffering (default).
►
Buffering through the ADA4807-2
►
necessary if additional load is placed on the AD4080 CMO
output. Note that this output has an output impedance of 700
Ω; consult the AD4080 data sheet for additional details.
EVAL-AD4080
LT6657 LTC6655
Yes No
Yes No
Yes Yes
Yes No
Yes No
1.2 5
−40°C to +125°C −40°C to +125°C
5 3
REF
ADA4945-1
and +V
CLAMP CLAMP
(A5) amplifier, which is only
Rev. 0 | 5 of 19
)/2,
).