DAC900-Q1 Datasheet, PDF (17/26 Pages) Texas Instruments – 10-BIT 165-MSPS DIGITAL-TO-ANALOG CONVERTER

DAC900-Q1
www.ti.com
SBAS505 – JUNE 2010
Dual Transimpedance Output Configuration
The circuit example of Figure 5 shows the signal output currents connected into the summing junction of the
OPA2680, which is set up as a transimpedance stage, or I-to-V converter. With this circuit, the DAC's output will
be kept at a virtual ground, minimizing the effects of output impedance variations, and resulting in the best DC
linearity (INL). However, as mentioned previously, the amplifier may be driven into slew-rate limitations, and
produce unwanted distortion. This may occur especially at high DAC update rates.
+5V
50Ω
1/2
OPA2680
–V_OUT = I_OUT • R_F
DAC900
I_OUT
C_D1
I_OUT
C_D2
R_F1
C_F1
R_F2
C_F2
1/2
OPA2680
–V_OUT = I_OUT • R_F
50Ω
–5V
Figure 5. Dual Voltage-Feedback Amplifier OPA2680 Forms Differential Transimpedance Amplifier
The DC gain for this circuit is equal to feedback resistor RF. At high frequencies, the DAC output impedance
(CD1, CD2) will produce a zero in the noise gain for the OPA2680 that may cause peaking in the closed-loop
frequency response. C_F is added across R_F to compensate for this noise-gain peaking. To achieve a flat
transimpedance frequency response, the pole in each feedback network should be set to:
1
GBP
=
2pR_FC_F 4pR_FC_D
(8)
Where,
GBP = Gain Bandwidth Product of OPA
This gives a corner frequency f_-3dB of approximately:
GBP
f_-3dB = _2pRFCD
(9)
The full-scale output voltage is defined by the product of I_OUTFS × R_F, and has a negative unipolar excursion. To
improve on the ac performance of this circuit, adjustment of R_F and/or I_OUTFS should be considered. Further
extensions of this application example may include adding a differential filter at the OPA2680's output followed
by a transformer, in order to convert to a single-ended signal.
Copyright © 2010, Texas Instruments Incorporated
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