THS4304 Datasheet, PDF (21/26 Pages) Texas Instruments – Wideband Operational Amplifier

THS4304
www.ti.com
SLOS436A – MARCH 2004 – REVISED JULY 2004
NON-INVERTING SINGLE-SUPPLY OPERATION
The THS4304 EVM can easily be configured for single 5-V supply operation, as shown in the following
schematic, with no change in performance. This circuit passes dc signals at the input, so care must be taken to
reference (or bias) the input signal to mid-supply.
If dc operation is not required, the amplifier can be ac coupled by inserting a capacitor in series with the input
(C7) and output (C9).
V_REF
R3
10 kW
R6
+V_S
10 kW
−V_S GND GND +V_S
J3 J4 J6 J5
NC
+V_S
FB2
C8
R8
R7
0.1 mF
249 W
249 W
GND
TP1
C2
3.3 mF
+V_S
J1
C7
R10
0
0
44 _−
3_{3 +}
55^U11₁
22 ^THS4304DBV
R2
49.9 W
C9
0
J2
R1
49.9 W
C5
0
V_REF
C4
0.1 mF
Figure 54. Non-Inverting 5-V Single-Supply Amplifier
DIFFERENTIAL ADC DRIVE AMPLIFIER
The circuit shown in Figure 54 is adapted as shown in Figure 55 to provide a high-performance differential
amplifier drive circuit for use with high-performance ADCs, like the ADS5500 (14-bit 125-MSP ADC). For testing
purposes, the circuit uses a transformer to convert the signal from a single-ended source to differential. If the
input signal source in your application is differential and biased to mid-rail, no transformer is required.
The circuit employs two amplifiers to provide a differential signal path to the ADS5500. A resistor divider (two
10-kΩ resistors) is used to obtain a mid-supply reference voltage of 2.5 V (VREF) (the same as shown in the
single-supply circuit of Figure 54). Applying this voltage to the one side of RG and to the positive input of the
operational amplifier (via the center-tap of the transformer) sets the input and output common-mode voltage of
the operational amplifiers to mid-rail to optimize their performance. The ADS5500 requires an input
common-mode voltage of 1.5 V. Due to the mismatch in required common-mode voltage, the signal is ac coupled
from the amplifier output, via the two 1-nF capacitors, to the input of the ADC. The CM voltage of the ADS5500 is
used to bias the ADC input to the required voltage, via the 1-kΩ resistors. Note: 100-µA common-mode current is
drawn by the ADS5500 input stage (at 125 MSPS). This causes a 100-mV shift in the input common-mode
voltage, which does not impact the performance when driving the input to –1 dB of full scale. To offset this effect,
a voltage divider from the power supply can be used to derive the input common-mode voltage reference.
Because the operational amplifiers are configured as non-inverting, the inputs are high impedance. This is
particularly useful when interfacing to a high-impedance source. In this situation, the amplifiers provide
impedance matching and amplification of the signal.
The SFDR performance of the circuit is shown in the following graph (see Figure 56) and provides for full
performance from the ADS5500 to 40 MHz.
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