ADC12D1000_15 Datasheet, PDF (60/86 Pages) Texas Instruments – GSPS Ultra High-Speed ADC

ADC12D1000, ADC12D1600
SNAS480N – MAY 2010 – REVISED AUGUST 2015
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8.1.1.2 Driving the ADC in DES Mode
The ADC12D1x00 can be configured as either a 2-channel, 1.0/1.6 GSPS device (Non-DES Mode) or a 1-
channel 2.0/3.2 GSPS device (DES Mode). When the device is configured in DES Mode, there is a choice for
with which input to drive the single-channel ADC. These are the 3 options:
DES – externally driving the I-channel input only. This is the default selection when the ADC is configured in DES
Mode. It may also be referred to as “DESI” for added clarity.
DESQ – externally driving the Q-channel input only.
DESIQ – externally driving both the I- and Q-channel inputs. VinI+ and VinQ+ should be driven with the exact
same signal. VinI- and VinQ- should be driven with the exact same signal, which is the differential complement to
the one driving VinI+ and VinQ+.
The input impedance for each I- and Q-input is 100 Ω differential (or 50 Ω single-ended), so the trace to each
VinI+, VinI-, VinQ+, and VinQ- should always be 50 Ω single-ended. If a single I- or Q-input is being driven, then
that input will present a 100-Ω differential load. For example, if a 50-Ω single-ended source is driving the ADC,
then a 1:2 balun will transform the impedance to 100 Ω differential. However, if the ADC is being driven in DESIQ
Mode, then the 100-Ω differential impedance from the I-input will appear in parallel with the Q-input for a
composite load of 50 Ω differential and a 1:1 balun would be appropriate. See Figure 66 for an example circuit
driving the ADC in DESIQ Mode. A recommended part selection is using the Mini-Circuits TC1-1-13MA+ balun
with Ccouple = 0.22 µF.
50:
Source
1:1 Balun
C_{couple VINI+}
100:
C_couple ^VINI-
C_{couple VINQ+}
100:
C_couple ^VINQ-
ADC1XD1X00
Figure 66. Driving DESIQ Mode
In the case that only one channel is used in Non-DES Mode or that the ADC is driven in DESI or DESQ Mode,
the unused analog input should be terminated to reduce any noise coupling into the ADC. See Table 27 for
details.
MODE
Non-DES
DES/Non-DES
DES/Non-DES
Table 27. Unused Analog Input Recommended Termination
POWER DOWN
Yes
No
No
COUPLING
AC-DC
DC
AC
RECOMMENDED TERMINATION
Tie Unused+ and Unused- to Vbg
Tie Unused+ and Unused- to Vbg
Tie Unused+ to Unused-
8.1.1.3 FSR and the Reference Voltage
The full-scale analog differential input range (V_{IN_FSR}) of the ADC12D1x00 is derived from an internal bandgap
reference. In Non-ECM, this full-scale range has two settings controlled by the FSR Pin; see Full-Scale Input
Range Pin (FSR). The FSR Pin operates on both I- and Q-channels. In ECM, the full-scale range may be
independently set for each channel through Addr:3h and Bh with 15 bits of precision; see Register Maps. The
best SNR is obtained with a higher full-scale input range, but better distortion and SFDR are obtained with a
lower full-scale input range. It is not possible to use an external analog reference voltage to modify the full-scale
range, and this adjustment should only be done digitally, as described.
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