Datasheet

3 Analog Interface
3.1 Signal Conditioning
V
I
R20
300 W
R24
300 W
-VCC
1 Fm
1 Fm
C5
C3
1 Fm
C18
680pF
C20
THS4031
U4
+2.5V
R13
10kW
R5
0 W
R25
15 W
+VCC
VIN
www.ti.com Analog Interface
The ADS788x ADC has one analog input pin. A signal for the input pin can be applied at connector P1,
pin 2 (as shown in Table 2 ), or applied to the center pin of SMA connector J1. The input range of the
converter set by the power-supply voltage applied at pin 1. For example, if VDD = 2.35 V, then V
I
can
range from 0 V up to 2.35 V or 2.35 V
PP
.
Table 2. Analog Input Connector
Description Signal Name Connector and Pin Number Signal Name Description
Reserved N/C P1.1 P1.2 ( + ) VIN
Reserved N/C P1.3 P1.4 N/C Reserved
Reserved N/C P1.5 P1.6 N/C Reserved
Reserved N/C P1.7 P1.8 N/C Reserved
Pin tied to Ground AGND P1.9 P.10 N/C Reserved
Pin tied to Ground AGND P1.11 P1.12 N/C Reserved
Reserved N/C P1.13 P1.14 N/C Reserved
Pin tied to Ground AGND P1.15 P1.16 N/C Reserved
Pin tied to Ground AGND P1.17 P1.18 N/C Reserved
Reserved N/C P1.19 P1.20 EXT_REF External Reference Input
The VIN input to the ADS788x is driven with a low-impedance source. In many cases, an input driver is
not necessary; however, in cases where the source impedance exceeds 200 Ω , using a buffer helps
achieve the rated performance of the converter.
The amplifier circuit shown in Figure 1 is the buffer circuit used on the ADS788xEVM. This circuit consists
of the THS4031, a high-speed, low-noise, low-distortion amplifier configured as an inverting gain of 1. The
circuit shown Figure 1 is optimized to achieve the ac (that is, SNR, THD, SFDR, etc.) specifications listed
in the respective ADS788x product data sheets. Note that the input circuit may require adjustments to
achieve best performance for the test system.
Figure 1. Input Buffer Circuit
SLAU166B – October 2005 – Revised August 2008 ADS788xEVM 7
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