THS4521 Datasheet, PDF (26/43 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521
THS4522
THS4524
SBOS458B – DECEMBER 2008 – REVISED MAY 2009.................................................................................................................................................... www.ti.com
Setting the Output Common-Mode Voltage
The output common-model voltage is set by the
voltage at the V_OCM pin. The internal common-mode
control circuit maintains the output common-mode
voltage within 5-mV offset (typ) from the set voltage.
If left unconnected, the common-mode set point is set
to midsupply by internal circuitry, which may be
overdriven from an external source. Figure 62
represents the V_OCM input. The internal V_OCM circuit
has typically 23 MHz of –3 dB bandwidth, which is
required for best performance, but it is intended to be
a dc bias input pin. A 0.22-µF bypass capacitor is
recommended on this pin to reduce noise. The
external current required to overdrive the internal
resistor divider is given approximately by the formula
in Equation 2:
I_EXT =
2V_OCM - (V_S+ - V_S-)
50 kW
(2)
where:
• V_OCM is the voltage applied to the V_OCM pin
V_S+
Single-Supply Operation
To facilitate testing with common lab equipment, the
THS4521EVM allows for split-supply operation; most
of the characterization data presented in this data
sheet is measured using split-supply power inputs.
The device can easily be used with a single-supply
power input without degrading performance.
Figure 63 shows a dc-coupled single-supply circuit
with single-ended inputs. This circuit can also be
applied to differential input sources.
V_IN+
PD Control
Optional;
installed to
balance
impedance seen
at V_IN+
R_G
R_IT
R_F
V_S+
PD
0.22 mF
THS452x
V_S-
V_OCM
R_IT ^RG
R_F
R_O
V_OUT-
R_O
V_OUT+
V_OCM Control
0.22 mF
To internal
V_OCM circuit
100 kW
100 kW
I_EXT
V_OCM
V_S-
Figure 62. V_OCM Input Circuit
Typical Performance Variation with Supply
Voltage
The THS4521, THS4522, and THS4524 family of
devices provide excellent performance across the
specified power-supply range of 2.5 V to 5.5 V with
only minor variations. The input and output voltage
compliance ranges track with the power supply in
nearly a 1:1 correlation. Other changes can be
observed in slew rate, output current drive, open-loop
gain, bandwidth, and distortion. Table 3 shows the
typical variation to be expected in these key
performance parameters.
Figure 63. THS4521 DC-Coupled Single-Supply
with Single-Ended Inputs
The input common-mode voltage range of the
THS4521, THS4522, and THS4524 family is
designed to include the negative supply voltage. in
the circuit shown in Figure 63, the signal source is
referenced to ground. V_OCM is set by an external
control source or, if left unconnected, the internal
circuit defaults to midsupply. Together with the input
impedance of the amplifier circuit, R_IT provides input
termination, which is also referenced to ground.
Note that R_IT and optional matching components are
added to the alternate input to balance the
impedance at signal input.
Table 3. Typical Performance Variation versus Power-Supply Voltage
PARAMETER
–3-dB Small-signal bandwidth
Slew rate (2-V step)
Harmonic distortion at 1 MHz, 2 V_PP, R_L = 1 kΩ
xxxSecond harmonic
xxxThird harmonic
Open-loop gain
Linear output current drive
V_S = 5 V
145 MHz
490 V/µs
–85 dBc
–91 dBc
119 dB
55 mA
V_S = 3.3 V
135 MHz
420 V/µs
–85 dBc
–90 dBc
116 dB
35 mA
V_S = 2.5 V
125 MHz
210 V/µs
–84 dBc
–88 dBc
115 dB
24 mA
26
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